"The smallest rifle I have is a 223... "
Whats the next smallest?
For a small framed, short armed person, I'd lean more
towards what fits and feels good to them.
Shoots some targets with whatever guns you think might work.
Might surprise you what he can shoot.
I know the .223 would work fine for most shots (assuming its legal).
Several of my peers use various .22hi-powered rounds.
Figure that most deer are well within 100yds when shot.
For me though, I just don't want to face the biggest deer of my lifetime, presenting a poor shot angle(for caliber), with the smallest gun in my cabinet. I know that's how it'd happen for me.