INgal,
Hi there. This is the second time that I've seen that you are concerned that your AS' future plans are not realistic. You might be the first to pick disordered thinking up. Are you possibly seeing signs of a secondary diagnosis? I guess it is hard to know how far out there these plans are, but you are concerned. I know last week he wanted to be a MD.
That is a tough question I think, "how to support future plans but not unrealistic plans."
As a parent, this question is best animated in "Rudolph the Red Nosed Reindeer." Hermie the elf wants to be a dentist. But he is stuck making toys for a living. Boy is he a thorn in his Elf supervisor's suspender. Clearly he must have studied dentistry off camera though, because he was ready! Who would ever think the abominable snow man would someday be his patient!? And that one famous patient segued into a life of caring for elven teeth.
We just don't know entirely what paths our lives will take. So maybe the best approach is to ask questions about the future goals and why it calls to him. Ask him how he would pursue these goals. And just listen without criticism or suggestions or a Google search of schools with that program or a big story about someone famous who made it in via an alternative path. Just be.