I believe that when you have a deductable (whether high or not), that must be met before they kick in any payments.
Your son really needs to do his homework and find out exactly what his coverage is, and if he discovers that he will have to pay most out of pocket, then the cheapest way would be an overnight home study. Then start checking online for a reasonably price APAP. (Not a Cpap).
He can see a regular GP or dentist, but sometimes they are not that keen on writing a script for anything Cpap related. My dentist stated he didn't know much about Cpap and was not comfortable with writing a script, and my GP said much the same.
I had to see a sleep specialist.
Anyway, as I said, you son needs to check first on his coverage.
Good luck in helping your son maneuver the Heath Care system.