You mean, “How did he choose dorm?” I’m not a mom (or dad) - I’m the gay son. My mom had no voice in how I chose my dorm (tbh, neither did I, since at my college, freshman dorm assignments are random, but you get the idea).
That said, I agree with @wis75. I’ve had absolutely no issues at all with my roommates and dormmates, either gay. straight, male, female, black, white, blue. I would assume it’s also a non-issue at UCB.
Everyone is unique, but it sounds like your son is doing fine. Being gay is just one part of my identity, and a small one at that. While I do have gay friends, I choose my friends for who they are as a person. So there is no set percentage of gay friends that anyone would/should have. Also note that he will probably come into more gay people (by a large percentage) at UCB than he did in his hometown.
I’m not a UCB expert, so another parent/student can help, but a quick google search revealed a lot of LGBTQ resorces there.
http://ejce.berkeley.edu/geneq/lgbtq-resources
With these, he should be able to navigate his way through.