If he doesn't have much experience behind the trigger, get him a .22 long rifle. It's a cheap and easy way to get a lot of trigger time, and there is no noise and no recoil to cause him to develop any bad habits. After he gets a little more comfortable shooting and has done some hunting with a .22, then let him graduate to a centerfire rifle.

As far as rifle calibers are concerned, my pick would be a .25-06. It shoots very flat, and is capable of stabilizing a 110 - 120 grain bullet for deer hunting. Recoil is still very mild in most rifles, but that little extra bullet weight makes it a little bit more forgiving than a .243.