It' s tough to buy speculative gifts for bowhunters, since we are a picky lot when it comes to the equipment we use. I know that I find no use in 99.99999999% of the bowhunting " products" on the market.
Perhaps, rather than trying to buy something specific for his bow, you could consider something useful to bowhunting in general.
Does he have a desire to make his own arrows? A Bitzenberger fletching jig (tool used to mount feathers on the bare arrow shaft) would be fantastic if he does. ~$85
Does he butcher his own deer, or have a way of hanging it? A skinning gambrel & hoist (used to lift and hold the deer for skinning/quartering) would be perfect. ~$30
Does he hunt from the ground? A Nif-T-Seat is a fantastic portable/compact/quiet seat for bowhunters. (I' ve tried a BUNCH of portable hunting seats and this is the only one I like.) ~$50
Another neat item would be to get him a custom knife...something that would require more effort to find, but would be special and unique. I have two that I treasure...a curly maple handled neck knife with hand forged blade and leather neck sheath, and an African Leadwood handled knife with a blade forged from an old sawmill blade, and etched with a " cave painting" of a caveman bowhunter shooting a mountain goat. Here' s a picture of the latter. The maker, Jim Sigg, is in West Jordan, Utah. His phone/fax number is (801)-566-0050 if you want to reach him.