What would be the best branch of service for my son?

I have served in all 3 branches Active and Reserve. I would advise him to join the Air Force Reserve. If he likes it
..go active...if not...just one weekend a month..... Air Cargo Aerialport is a good AFSC...mos...easy. Can travel the World.

I encourage your son to talk with recruiters from all 5 armed services, Army, Navy, Marines, Air Force, and Coast Guard. The level of discipline and supervision differs from Service to Service and among specialties within each Service. Here's my opinion of the Services based only on my experience. Caution: very broad generalities abound.
-Air Force. The Air Force has a warfighting approach that differs from the other Services. The officers (pilots) are the front-line troops taking fire and steel to the enemy. Most of the enlisted personnel work supporting aircraft and aircrews and logistics infrastructure necessary to keep them flying. The Air Force has some personnel that work with space systems and provide Security Forces (somewhere between beat cops and light infantry). Because of the highly technical nature of much of the work done by enlisted personnel, there generally is a more business-like environment in the Air Force. Junior enlisted are often required to do technical tasks, such a repairing a jet engine, with moderate to light supervision. Others, such as enlisted aircrew, may work with little daily supervision and in close coordination with officers. I served 22 years and it was a good life.
-Army. The Army has a wider range of occupations than the Air Force or Marine Corps. The Combat Arms provide our Nation's capability to operate effectively on the ground to fight and win. Junior enlisted Soldiers take the fight to the enemy under the direct leadership of Sergeants. Overall discipline is probably a little tighter than the Air Force or Coast Guard. Combat Arms includes Army Aviation, which has a warfighting paradigm similar to the Air Force--officers fly and fight, enlisted support helicopters. The Army includes specialties in almost any occupation you could think of and a lot you may not. A Soldier can work in logistics, intelligence, communications, IT, medical, legal, contracting, and many other MOS. The level of supervision varies, but mostly follow the Combat Arms model of junior enlisted groups or teams working under the supervision of a sergeant. I worked for 6 years as an Army Civilian in the transportation business. The officers and NCOs I worked with were excellent Soldiers.

-Navy. It's all about the sea. The Navy has a range of specialties wider then any other Service I think. Obviously they operate large ships, like aircraft carriers, all around the world. They also operate submarines and have their own air force (Naval Aviation). This all requires a large logistical infrastructure on land and at sea. Junior enlisted sailors work under supervision of petty officers. Discipline is tight aboard ship or submarine. Ashore units may be more relaxed, but the structure of junior enlisted working for petty officers generally is the same. The Navy has all of the same logistics MOS available as the Army or Air Force. They provide logistics support functions for the Marine Corps.

-Marine Corps. The Marines are the Department of the Navy's ground forces. They do many important missions for the Navy and the DoD. They have Combat units similar to the Army, Infantry, Artillery, Aviation, etc., but differ greatly in numbers of units of any type. Discipline in the Corps is traditionally tight and Sergeants lead junior enlisted personnel to bring the fight from the sea to the enemy. Marines also work in logistics, communications, IT, and other support functions. Marines serving in "personnel other than grunts" jobs often work closely with Navy personnel to gain support for the Marines. Marines are very mission oriented. In my private sector management jobs, I would hire a former Marine over any other equally qualified applicant because Marines know how to get work done.

-Coast Guard. The Coast Guard is part of the Department of Homeland Security, but are still an armed service. They have most in common with the Navy, but have a law enforcement mission that the other four services aren't allowed to do. They are a small service compared to the others so they have limited opportunities, but have great missions. They sail a variety of ships and boats guarding the waters around and in the US. They also have an aviation department operating helicopters and a few fixed-wing aircraft. Their best known mission possibly is search and rescue. Their occupational opportunities are similar in variety to the Navy, but much smaller in number. I can't speak to their discipline or leadership philosophies, but expect they are much like the Navy.

This is his choice to make. Help him with his decision, but do not try to sway him to a particular branch or MOS. He needs to make his own choices in this matter. As for his job choice, I would say have him pick one that serves his long term goals.

Also consider what he's looking for if he were to deploy. Does he want to be on a ship? On an airfield? Boots on the ground? That is how he should pick his branch of service.

It really just depends on what kind of career he wants. Each branch has it's own individual perk. If he just wants to be a grunt and that's it, then Marines or Infantry is his game, however, it just depends on what he wants to do with his life...instead of thinking fives years in the future, think about twenty years in the future; what post military career does he want? what is his passion? Think of it that way and the answer of what branch and MOS will come to him. Example: I've always loved law enforcement and I plan on moving in to FBI or DEA, so, being an MP was my choice of career in the Army.

I would vote for the Marines.

I beg to differ on someone needing help from a parent to make their children's decisions about where they will feel better suited to serve. You might be making a decision that could make or break the person. Would you like to be the person's decision as to commit suicide or not if they can't fit in? That should be something to consider. If you want your child to get it together by joining the service, maybe the thing to do is encourage them to join the service of their choosing, and if you wish to influence them, then perhaps advise them on the job they want to do. The ASVAB will also be a deciding factor as to the service pick and the various jobs they get in the military. Please do yourself a favor and encourage them towards what the ASVAB dictates.

Sir, I can not say which would be the best for your son, but I can offer a little hope.
My nephew is technically on the spectrum, however sharing some of your son's characteristics did not prevent him from becoming a Sergeant in MOS 19D Cavalry Scout. He served multiple tours in Southeast Asia and now lives happily with a wife and children in Texas.

Almost a year later and my son is heading out for Navy bootcamp and then to AIT as Machinist mate. The Covid virus has reduced bootcamp to 2 weeks quarantine and 5 weeks bootcamp.

My son has an appt with the Army recruiter tomorrow morning. I'll then set up an appt for the Marine recruiter next week.