Many colleges require a minimum GPA to keep scholarships. They all have a minimum GPA to remain in good standing. I’d tell him if his grades drop, he’ll be deferring acceptance and working until he’s mature enough to handle college. A drop in grades could get his acceptances rescinded anyway, so you may not have to worry about making him defer. The colleges may take that decision out of your hands.
I’d drop discussions about transferring to UCLA because it’s going to be more difficult for him to move on if you don’t. It’s understandable that he’s disappointed but he needs to figure out a new plan, and I think it’s important that you make your expectations clear to him while he does that. Are you willing to pay top dollar if he refuses to put forth his best effort? Can you afford to cover any scholarships if he loses them? If he starts at a community college (which makes him a transfer student and ineligible for freshman grants), can you be full pay the last 2 years? Not trying is a choice, and it comes with consequences. You need to let him know what those are.