Since you didn't give any specific info regarding what the rifle will be used for, I'm going assume that, since you are on a deer-hunting site, this will be the application. As a general rule, I would pick the smallest caliber that would get the job done humanely, especially since you said he was on the smallish side (which, by the way, is my size as well). I would avoid dampening his enthusiasm (or causing a flinch to develop) by getting a rifle which doesn't beats the crap out of him. Here are my suggestions: nothing larger than a 24 caliber/6mm - a .243 Win. should fill the bill; a bolt action which is easier to operate and somewhat safer than other actions (unless you go for a single shot); also get a rifle with an easy to use/"positive" safety; get a good recoil pad, like a Limbsaver or Kick-eez. Don't start him thinking that he will be more "manly" if he shoots a big gun - just get him to have fun and shoot straight and be safe. The "magnums" can come later, if ever.