MWM, just from experience and not going by IDEA to the "tee", the transition plan should identify what your son wants in terms of post high school (living situation, job situation), what skills he needs (budgeting, cooking, etc.), what supports he might need (independent living, supported living, supervised living, and ditto job), what agencies will/might be involved post HS (here in IL I think the agencies are actuallly supposed to, like, show up at IEP mtgs but pigs will fly before that happens here and I believe the new IDEA relieves districts of insuring that happens if they "try"), and who will be responsible to insure he gets the post HS skills he needs. The new IDEA is more strongly worded (if you have a compliant state) in that the transition goals must be "results oriented" or something along those lines. I think in practice it gives parents a leg up if the kid will need education beyond age 18. I seem to also recall that with Boo, the transition specialist did an inventory of hobbies, likes, recreational/social wants, etc.
Because of Boo's severe involvement (needing 1:1 24/7 care) and because I know that I literally will have to die before Boo gets any services in IL, I blow off his transition plan. We'll make our own arrangements for programming post HS. With thank you, it's a moot point because he's beyond uncooperative.
If you've got a district that offers life skills programming or other options (voc. ed. for example), I think it's a useful tool. This also might be the prime time for you to lay the groundwork for a voc. ed. evaluation if you're not sure what your son can or might be able to do or how independently. Boo had a simply bang up evaluation, after much SD kvetching, at a local rehab hospital.
I do know of a parent here in IL who has a son who was in Residential Treatment Center (RTC) for years, returned home, and somehow Dad (who is one heck of a warrior parent) got the SD to pay for vocational school after age 18 based on IEP and transition plan goals. Pretty phenomenal.
Good luck - I hope it's a good process for you and your son.