My daughter’s abroad right now, and we dealt with these issues.
1.Phone plan - do you have Verizon, by any chance? If so, switch to the unlimited ultimate plan, which allows unlimited talk/text/data while abroad. You don’t have to make any other changes. If not, see if your carrier has something similar. My daughter has had no interruptions in her service, has not had to get a local SIM card, change her umber, etc. However, if you don’t have a workable phone plan, those solutions will wirk.
2.Meds: Call your insurance company and explain the situation. They can arrange for a suspension of the usual rules for refills so he can get what he needs before he goes. Either they can enter the exception into the system, or they can permit your son’s doctor to prescribe more than usual. In our case, we got a week’s grace period to get four months’ worth of meds, and when our pharmacy ran out of one of the prescriptions, we got the grace period extended so we could find a different pharmacy. It all worked out. Your son will have to hand-carry those meds (i.e., in carry-on bag, not a checked bag). But you should be fine.
Don’t try to find a provider to prescribe meds abroad – we were warned against that – although he might be able to find a mental health provider for therapy, etc. And you are correct that meds cannot be mailed.