Have any of you made the switch from Navy to Marines? What was your experience, and is there anyway to keep your rank?

Honestly, from a practical standpoint, I wouldn't make that particular switch. If you're going to switch branches, go Army. Just by virtue of size and budget, the Army can simply do more for him, you and your family. Honestly, that's the smartest switch anyone can make.
Source : I did it too! (USMC 99-04, Army 05-current)
The Army has:
Better bases
Better diversity of assignments
Better schools
More schools
Better training
More training opportunities
Better Promotion potential
More diverse mission types
More broadening opportunities
More overseas assignments
More deployment opportunity
Better access to tuition assistance
More family support
Less stingy awards
Better civilian credentialing
Better Equipment
The list goes on and on and on and on.... The Marines are basically K Mart to the Army's Target.

Hopefully whatever he was medically discharged for has been resolved. If he was a MA2 and discharged in 2010, how old is he now? You have to been in boot camp before your 29th birthday. There is no getting around being a E-2, there are a select few who get E-3 out of bootcamp, but those are 1 in 500 or so and normally reserved to the top boot camp graduate. His promotion up the ranks to E-4 and E-5 will be accelerated because it is based off a point system and he has significant points for time in service which will put him ahead of his peers at the same time. There has to be a job opening in the MP field for him to get it. He also has to have the line scores from his ASVAB to qualify. Not all branches determine jobs the same way and have the same line score determination. For example, a individual I put in was qualified for firefighting in the Air Force and wanted to do it in the Marine Corps. He was not qualified to do the exact same job in the Marines. Same exact school which is run by the Air Force, different determinations. My intent is not to sound negative if it comes off that way, just some things I have seen before as both a MP and a Recruiter. The military police field is similar to being a MA. There is no ship security per se. Two different sub sections within the MP field. Garrison (base operations, gate guard, road patrol, etc) Field side ( Combat Arms side. Convoy security, Enemy Prisoner of war, route reconnaissance, tactical site exploitation, area security) There is a larger group on the field side than the Garrison side. The MP force is blended with civilian police officers on the garrison side. You do not get a choice which you do, you just get told. Some stay field side, some stay garrison, some do both side in a tour. As far as the dependent waiver, I cannot remember the rules, however I believe there is certain exceptions to the waiver for prior service as opposed to a young individual with no prior service. Best of luck to you.

The Few the Proud