In no particular order:
DeFeet wool socks. (they come in both summer and winter weight)
Cycling gloves. (every rider goes through gloves)
Cycling sunglasses.
Blackburn Flea rear LED tail light. (the smallest, brightest, lightest, and rechargeable)
Pearl Izumi shorts or bibs.
Gift card at your local bike shop. (you know the one - his favorite)
A ding dong bell. (just to be silly)