Just stumbled on this thread in a search for the word "music".
My sympathies.
I skimmed all the posts, and they covered almost everything. Some points I didn't hear anyone else making:
-If your son hasn't had any flashbacks by now, he's not going to have them.
The whole "Manchurian Candidate" thing or walking-timebomb thing is a complete myth. If your son has flashbacks (which do occur with some people) he would know it already. For some people, mild flashbacks occur and they simply aren't troubling. (I'm thinking of Ken Kesey, who said he could always tell his flashbacks from reality.)
-Creepy old W. Burroughs made the point somewhere that, for some people, LSD has the lasting effect of making them more aware or self-conscious about their perceptions, because, prior to taking LSD, they simply took perception for granted. So the effect is a matter of learning something about the potential instability of one's own mind. A useful lesson? Burroughs thinks it can permanently open up a kind of sensitivity to art. I honestly don't know.
-I agree strongly that the best course of action is to lie. Since I hate to lie, this would put me in a bind. I wouldn't apply for this kind of job. I wouldn't be a candidate for the Supreme Court, either.
-Maybe your son's best real contribution could still be as a musician. It takes all kinds.
-Some people have been completely derailed by LSD (Syd Barrett, for example) and some people I know. This is MUCH more likely if they made the stupid mistake of taking a high dose. John Lennon survived much high dosing because he had a high tolerance from taking LSD every day. After 4 or 5 days, the LSD had virtually no effect. It's not clear to me whether the people who got "derailed" had pre-existing dispositions to mental illness.
Just my anecdotal 2 cents. Sorry if I missed something in the other posts.