I know what placement test they use because I'm a student at the same school. It's multiple choice for math, and an essay for writing. I don't know how you can test for writing without actually having a person write....? They want to know if they can stick you in Comp 101 or if you need remedial work. Knowing grammar and punctuation is not the same thing as being able to compose.
need help with adult child - Page 2
Anyway, it sounds like his anxiety is the bigger problem, then? I think he could get the help he needs with his writing at a community college remedial writing class (and a class like that would probably be set up to help kids who are anxious about having to write, too), but he can't get there without taking the placement test, and his anxiety about writing prevents him from doing that. So it's a catch-22 of sorts.
Is his therapist addressing his anxieties at all? Has he ever been tried on a low-dose anti-anxiety medication like xanax? Those medications got a bad rap because of their potential for addiction, but I know a number of people who found that a low dose for a temporary time period was really useful in helping them get past stumbling blocks like this. YMMV, of course, and I'm not a doctor...
Google around for SAT essay tips (even if he's not going to take it). There are some good (free) sites that give very nuts and bolts, practical advice on how to write good essays and the steps involved. Given that people only have about 30 minutes to write the essay, it's a nice way to have a crash course.