I agree, a gift card to his favorite store would be a good choice! It seems kind of lame, but backpacking stuff is expensive, and I have a list of very specific stuff I am planning on getting the next time I have enough money.
Otherwise, if you want to get him something, here are a few things he might like:

*GPS - Expensive, but I guarantee he would be ecstatic about it! This is on my list
*MSR Groundhog Tent Stakes (lighter=less pounds)
*Headlamp - Way more useful than a flashlight. (I have a Princeton personally)
*Anything that is new and cool/latest gear