if you know what he's already tried and likes, go off of that and try to branch out a little into something new, so if he's already tried different vodkas or rums, maybe look at scotch as already suggested (Macallan and Glenmorangie are a couple of my favorites), usually the younger ones, 10-12 year, are easier for a first exposure than the older, smokier ones..... or bourbons (four roses, elijah craig, for a couple examples)
if you go with tequila (and he hasn't had much exposure to it), then look for a sampler pack, where you can get 3 versions (silver, reposado, anejo) in smaller bottles to try.... (actually, you can find the sampler pack type deals in a lot of different alcohols; which is nice since you can try a few different types and not be stuck with a large bottle of something you might not really like)
might also look at a bartending book, if he doesn't have one; so he'll have a good mix of recipes and the ins-n-outs of mixing drinks.