I high school I was a utility player; trumpet, baritone, and french horn, sometimes all in the same concert. I found the switch from trumpet to baritone pretty easy and it didn't affect my trumpet playing. Just make sure that whatever he chooses, he splits his practice time. Learning a new embouchure shouldn't ruin his current one as long as he practices both and learns to feel the differences. I have heard some people say that playing a different instrument actually improves their tone on their regular horn.