How about something he won't expect? A birthday message in the local paper, a silver love heart or name a star after him?
www.buyagift.co.uk. They also have a message in a bottle etc etc.
If he's really into computers, I would suggest against getting him anything computer orientated, especially if you're NOT into computers, as you'd probably get the wrong thing, and these computer nerds have seen it all before etc etc.
How about a trip to game expo, or if you can, a trip to see a tribute band to one of his faves.
Good luck, whatever you choose!