It all depends on what he collects.
Does he collect copper? Get him a Large Cent or a
Half Cent, a lovely BU early
Lincoln Cent or a Unc later-date
Indian Head cent.
Does he collect silver coins? Get him a nice MS-63 or MS-64
Morgan dollar or
Peace dollar.
Does he collect gold coins? Get him a 1/10 ounce American Gold Eagle (a bit more than $100), or a 1/20 ounce gold coin (you will be able to find many different ones for less than $100).
Does he collect Commemorative coins? Get him a
Baseball Hall of Fame commemorative dollar+half dollar, or a nice BU Columbian half dollar or a BU common classic commemorative coin.
Does he collect silver bullion? Get him some silver bullion coins, like the American Silver Eagle or the Canadian Maple Leaf.
Does he collect proof sets? Get him a 50s Proof Set or the current 2014 Silver Proof set.
The possibilities are endless.
I think you should find out what he likes to collect to decide what to buy for him. That way it would be the perfect gift. :)