A dozen of the ball he already plays
Look in the bag, read the name off the side of the ball, buy twelve of that. It is the most used golf gift there is and the only way to get it wrong is to substitute a ball you have heard of. Premium tour balls are roughly $50 to $60 a dozen, everything below that about $20 to $35.
Works because: he loses them faster than he replaces them, every year.