RE: What do you think?
First off, you are a good brother to take your younger brother hunting....
He should be fine, as long as you help him practice though out the summer...
My niece started hunting with her dad at 10-12, she killed her first deer at 12 with a 22-250 and the Federal Premium 60 grain Nosler Partition...She has killed 6 deer with this loading...Pick out a good deer bullet for him and he will be fine...
I have found that younger kids will listen more than when they get older...As you know, you will want to limit your shots to within 100 yards and 50 would be better...You might also need to setup a stand or blind just for him and adjust the rifle rest to his size...My brother even had his daughter in the stand in July-August shooting at targets at the distance he expected the deer to be at...All of this builds confidence, which is a good thing...Good Luck