My daughter took all the "big" tests in high school - APs, the SAT -- separately, in the guidance counselor's office. She had to since she has use of a laptop/computer as one of her accommodations for her arthritis.
But she always said it was much quieter and much less distracting and they even finished everything faster since there were so few kids!
I wonder if that might be an option for your son, if he has trouble focusing?
Just as a warning though - College Board will probably not give extra time for Crohn's, for tests like the SAT and for APs. Not unless you have a lot of documentation that really proves he needs it.
My daughter did not get it for her arthritis and she has had swollen fingers and trouble writing or even typing sometimes. However, she did not really need it so we didn't bother fighting it.