From having family members with substances issues, I know a lot of what triggers someone backsliding from substance sobriety is being around people with whom you’ve done it (those old friends and old places - your brain makes the connections with them and your vice of choice and makes it very difficult to resist). I know your son SAYS he won’t want to do substance free, but he should seriously consider it. Kids tend to make friends with people they live with, whether it’s a roommate, people on your floor or in your building. If you hook up with like-minded people, the ability to resist temptations will be easier. And not all of the kids in these substance-free dorms are being sequestered there by paranoid parents. Some just aren’t that much into the party scene, don’t want to hassle with the drunk/high people who come in late, loud, throw up, pass out, and/or require emergency services.
As everyone says, he will find weed ANYWHERE he goes if he wants it badly enough. To lessen the contact - and surround himself with people uninterested in it as well - try a substance-free dorm.