It does not sound like the problem just started. I like the idea of follow through but wonder if you have a "Plan B" in case he decides not to follow through and not do anything about college either. It sounds like he wants to go to college but if you make it more difficult to maintain who he wants to present himself as, he might not want that anymore and/or if he has a problem with college, as it sounds like he probably will, and drops out, then what do you have?

I would sit him down and tell him how you see his life and give him two or three scenarios for how the next couple years can go and let him choose: (1) evaluation, therapy, college, therapy must be for X years (2) evaluation, he gets a job and you contribute whatever you're willing to pay for college in that time to his support, therapy for Y period of time; (3/4) no evaluation/therapy, X support for Z period at home/not at home; etc.

I would seriously consider what you want. If he doesn't want to grow up/get help, I'd insist he move out and I would support him, at least as much as college would cost, for a period of up to a year or two. I'd set up a joint checking account so you could review the money, make strict rules when you'll put some in, etc., and, if he gets evaluation/therapy sweeten the pot and freedom but otherwise insist he get and maintain a job first or the support really goes down (like with divorce).