Building computers, shops at newegg.com and amazon.com - difficult to shop for - I can relate, that's exactly how I am.
My honest advice for this type of person is not to worry about surprising him with anything computer related because if he's like me, even if you knew he wanted a tablet, hard drive, pc oscilloscope adapter, voltmeter, or some other gadget - you're right - there are so many kinds that you'd better leave it up to him to pick the specific one to buy. For most of the computer parts/gadgets I buy, I do a lot of research to narrow it down to find the best item for my budget - there's no way my wife could ever get it right by surprising me - it's just a plain and simple fact.
Maybe just give him a newegg/amazon gift card, and let any surprises be some non-electronic items that you know he would like.
I also like Stella's idea, have him make a "wishlist" of specific items that he would like (specific brand, model, part number, etc), and then when an occassion comes up you want to get him a gift, surprise him with one of the items on his wishlist (and you might want to remind him to update the list as those occasions draw nearer).
Hope that helps.