There are so many hiking styles, from ultralight to heavyweight, that it's nearly impossible to answer your question without knowing a whole lot more about what kind of gear he already has and whether or not he's trying to change to a different style.

If he has a favorite gear store, particularly one where he's on a first-name basis with the staff, you might go there and ask them if he's been eyeing anything in particular.

If that doesn't work, I'd suggest a gift certificate at his favorite gear store. I'm in a similar boat as your boyfriend: I have very specific likes and dislikes, and a family that is well-intentioned but knows nothing about backpacking. I really appreciate gift certificates at the local outfitter more than an actual item, and over the years I've convinced them that a gift certificate is really a much more thoughtful gift than an actual thing.

If he doesn't have a favorite gear store, then a gift certificate to a major retailer (like REI, Moosejaw, or Backcountry Gear, among others) would also work. Click on the TLB Gear Portal on the upper right part of this page for links to some of the larger dealers. They carry a wide variety of stuff, so he'd probably be able to find something he wanted there.

Good luck, and remember, it really is the thought that counts. (Of course, if you'd learn to backpack, you wouldn't have problems like this in the future. )