I'm a grad student, not undergrad, but I got into all the UCs I applied to for undergrad, including Berkeley.
I'd pick one test--I did the SAT. My parents couldn't afford classes, so I checked the prep books out of the public library (for the main SAT I used Kaplan and Barrons but it's probably worth doing a search for which are considered best), went through all the material, and did all the practice tests. Then I checked out the other prep books and did the tests only from those ones too. I did better than my friends who took classes (though I guess I can't say I wouldn't have done better with a real class).
I think the practice tests are the most important part, and I'd recommend doing as many as possible--really doing them, not just looking at the questions and the answers. The other main thing is doing a little bit every day instead of trying to cram it all in at once. I think the real advantage of formal courses is not really anything about the material or the teaching, but the fact that they force you not to procrastinate. For some kids that might be really valuable.
Actually, if he's got time, I'd recommend a quick primer on learning strategies before getting into the studying. When studying, a lot of people spend their time on things that aren't actually all that effective. Though a bit long, there's a free (UCSD-based, actually) coursera course called "Learning How to Learn", as well as a youth version, though I have not done that one. Or, this mini article on learning myths is much shorter but also useful.