Go to a good gun shop (if you can find one) and find out what he likes and what fits him. Maybe he likes lever actions, maybe bolts. As for caliber, I'd say anything from 243 on up will work just fine, but magnums are not required for deer.

Also there are many used guns out there, and a lot of those are lightly used. Money saved on the rifle can be put toward optics. Spend at least 100 (200ish is better) on the optics.