Or entry into a race/event, but you'd need his running details for that (assuming he has a number)
Id be surprised if he didn't have most of the equipment he needed such as hi viz things, armbands etc. Things that need replacing after a while like socks are good, or the food/snack idea is a good one. A running CD like Now Running or whatever?
I know you said you'd like useful smaller items, but what about a voucher for a nearby running shop (like He Runs She Runs or Up & Running) to go towards some new running shoes (they're not cheap)