Can my son join the Air National Guard between his junior and senior year, and then go full time Air Force when he graduates?

The Air National Guard does not have split training option, but you can join after your junior year of high school, because the average wait time for school is so long.
That said, you cannot transfer to another component until training is complete.
If he graduates high school and is still waiting for his tech school date, he will still have to finish that before he is eligible to apply for a conditional release to be allowed to enlist in the active component. And the wait for a conditional release is 3-6 months.
He could be looking at getting to active duty about 18 months after high school on that path.
If the goal is Active Duty Air Force as soon as possible after high school graduation, then enlisting in the Air Force right after high school is much quicker.

Getting from reserve to active isn't easy. If wanting active duty just join active duty.

I know you asked about the Air Force however I know that the army takes high school kids their junior year (with parental permission of course) and they get paid until they graduate for contracting. They can go to BCT the summer before their senior year and then after graduating high school they will attend AIT.

When I joined the Air National Guard I was split training and came home for a month after basic before going to tech school. I was told I could have signed up between my junior and senior year with a guardian's signature but that was back in 2003. When I got out of the military they did have a special unit for those awaiting basic and tech school so they could do some form of training until school. However, back then Active duty was not taking Prior Service or even Air Guard members. There are a ton of questions that need to be asked and clarified before picking to go active or guard. For example I wasn't told before I joined that Active Duty has to pay in towards the GI Bill but Guard members don't and get the same GI Bill benefits.

To keep it short, no. The Air Force doesn’t work quite like the Army in that way. The [Enter Name of Your State Here] Air Guard has zero interest in bringing your kid in and paying for their training only to quickly release them to a different component. Would you? If Active Duty Air Force is the goal, just do that.

There is no split option for active duty if I was you I would have him wait until he is a senior in high school and then join to ship after school is
complete. Other option is to split option to guard then do a 368 conditional release to go active (not easy route). The reason why is guard and reserve are state programs active is federal.

Ignore this comment, i misunderstood the question. Unfortunately deleting comments isn't a function.