I think he should take the Davis acceptance and run with it. It’s a great school!
If he wants Berkeley, and it is his choice and ONLY choice, then he’s going to have to take the risk of maybe not being accepted yet again.
Our daughter chose Davis because of her campus visit. The students looked happy.
When she visited Berkeley, to consider applying there, she knew a number of former sports teammates who attended there. They did not appear happy, but stressed and anxious, in a very competitive environment and relayed information about the housing issues and stress.
Our daughter did not major in CS, but had extremely good housemates who were CS at Davis, then went on to work at Silicon.
Davis a very collaborative environment, even within the stress of the majors. Is it a Berkeley? No, but I don’t think it wants to be.
Plus, her friends who did attend Berkeley, would come to visit Davis, on the weekends to “chill” and get coffee at Dutch Brothers or walk to In-N-Out Burger or get sandwiches at Trader Joe’s.
My advice to you is to have your son visit Berkeley and Davis. Go to the CS offices and note the environment. Completely different both intrinsically and extrinsically.