Does your brother reload? That opens up more options. But Caliber isn't as straight forward without some other information.
But considering the following:
- The 270, 30-06 or 260Rem in Bolt Action is hard to beat... tried and true for Deer all the way up to Elk and Black Bear.
- Recoil Sensitive, then 6.5cm, 243Win, 7mm-08 or a 308Win.
- In AR-15 Platform, the 6.5 Grendel, 6.8SPC or 350Legend.
- If he loves Lever Action, then there is the 30-30. Which has possibly bagged more deer (and maybe small Black Bear) than any other caliber today. With the new bullet designs, its gets help to go a little faster with impressive expansion. If not recoil sensitive, then the 35Rem or 45-70 Gov.