My DH is retired Coast Guard and we both served in the Navy before he joined the Coast Guard. He switched because the Coast Guard has more "shore" billets (jobs) than the Navy and I wanted him to be around when the kids were born, etc., which he was. I can't think of a "down" side to the Coast Guard. The duty stations are mostly in the United States (not true for the other services), the Coast Guard is a smaller branch (as another poster said) so there is greater camaraderie and you get to really do things like save lives, stop drugs from entering the country, doing lots of search and rescue. Most of the military now does tend to want their people to go to school, so there's no reason he cannot work on a degree while he's in the Coast Guard. In addition to formal education, he'll get a wealth of on-the-job experience that will really be helpful when he gets out, if he chooses not to make the Coast Guard a career. It is hard when you're not familiar with what your son is talking about, but I hope it eases your mind a bit to hear from people on this board who have "been there, done that, got the T-shirt." LOL!
My own DS19 just joined the Navy. He's in "A" school now (training school after boot camp). Of course, since both his mom and dad are Navy vets, this seems like a natural choice, but I think he would also have done well and enjoyed being in the Coast Guard. Hope this helps!
-Dorothy (LadyZolt) (Navy vet, daughter of Army vet, wife of Navy/Coast Guard vet, mother of Navy son)