NCC has a very short program for dental assisting. One of mine is thinking about it. It might be a good option for your son, so he can see if he is really interested in eventually working as a dental hygienist. If he does this course and likes it, he might be able to reapply to Farmingdale, which my son wasn’t eligible for either; his grades are crummy because of dyslexia.
As for Westbury, I have just never heard anything good about it, except that the campus is pretty.