As a helicopter mom, the best thing you can do is nod and have little to no position. Your pro or con influence will only complicate things and will turn left or right and do so only 50/50% at best anyway. Your child got in and has to do something that is very hard. He will make a decision, and he will have no clue how to do this, and he will ask you to help.
I suggest you just be reflective and say "it sounds like you are saying", "I'm not you, but knowing you, you are saying that...". Just being neutral to psychiatry is putting you above average for a mom. Telling him not to join us will only ruin your credibility, or pigeon whole him into something he will not like and blame you for not warning him about.
This is all quick sand for you now. Psychiatry is a good occupation, a good life, and well compensated so be glad he is interested and stay out of it. If he ends up doing colonoscopies every half an hour from here on out, don't make that your call. Be happy he will do well and be happy. If he is less than happy, don't make that your problem.