Well, it really depends on what he already owns. You can always buy him some scent (doe in heat, dominant buck, etc.). Or, if you know that he doesn't have cover scents (which are VERY important), then you can buy him some human scent eliminating laundry detergent, soap, shampoo, and deoderant. Yes, the stuff works VERY well. I've had a doe walk right next to me (less than 5 feet away) and just look at me and wonder what the hell I was. If she couldn't smell me from that distance, then the stuff works.
If you know he has this sort of stuff, then you can get him things like a grunt call or rattling antlers.
Also, how does he hunt? Bow, shotgun, muzzleloader, handgun? Ammunition or heavily used accessories are always welcome. Things such as cleaning patches for guns or maybe some broadheads for the arrows.
Hope this helps.