My first reccomendation is to teach the one that is right eye dominant to shoot right handed. It will help a lot especially with long guns. If he's already shooting lefty, I think both of you will be amazed at the increase in accuracy by switching.

I shoot lefty, but only have one gun that is a left hand model. I tried a left hand 870 once and I'm so used to the way the safety is on a right hand model, I had a real hard time adjusting, so I just stayed with a righty. My lefty is an Anschutz 1416 .22. It's a bolt action.

I wouldn't rule out a bolt action just because of them being left handed, a lot of companies make left handed models now.

I think that the scope is just as important a choice as the rifle. A good scope makes a world of difference in my opinion.