It’s great that you’re in California! Tons of great options at the UCs, CalPolys, and CSUs! @Gumbymom’s UC Davis recommendations are awesome. I have a few others to toss in as you’re exploring.
One you mentioned — Rausser, at UCB — is excellent, especially with his forestry+science interest. Their Ecosystem Management and Forestry program could be a great fit, or if he goes another direction, he can still get a minor by going to Forestry Camp. Obviously, Berkeley is a reach for everyone, so it’s good to go into the process with eyes open. Biking and climbing are going to be a bit harder to do while at Berkeley, though there are some options in the general area, and the Cal Hiking and Outdoors Society seems active.
Another excellent option is at Cal Poly SLO, in their Natural Resources Management and Environmental Sciences department. Their BS’s include Environmental Earth and Soil Sciences, Environmental Management and Protection, and Forest and Fire Sciences. Cal Poly is all about “learning by doing”, so I expect there’d be a LOT of time in the field. I suspect a lot of their Forestry work might be geared towards industry, but looking into it will give you more insights. The Earth and Soil Sciences degree struck me as “geology + chemistry” when D22 was looking around; even if it’s not forestry-related, that might align with your son’s scientific interests.
A bit easier to get into would be Cal Poly Humboldt, which has a Forestry major with different concentrations that might appeal to him, like hydrology or soil science. As with the other Cal Polys, they really emphasize the “learn by doing” approach.