What kind of car(s) . If hes a real nut then get him a Gift Cert to JEGs or a place like that ( my defacto gift for my self when someone doesn't know what to get me) . It sounds like a cop out but getting him a car part (say a shifter or a tech or something like that ) Generally is a bad idea unless you know he wants a certain one.
If he likes to work on the car alot too then you can get him a
creeper
and a set of work gloves. As well as nice set of jack stands and hydraulic jack.
Lastly you can go to the local autobody shop and buy him a full out detail (usually ~300 )