I remember the three toys that stick on my head: The first was a Ninja Turtles Phoos Ball Table (I still own, just not have it set up, it still functions though), the second was a 200$ 3 foot long Lego Space Ship, and the third was The Great K’nex Ferris Wheel.
Why do I remember all these? Because my mom played with me for days after Christmas, building those things, and playing Phoose Ball.
I say if you are looking for something that will be special to him, get something he can enjoy with you. Buy him a big set of Lego’s (not Star Wars, but just get a big bag of them off ether eBay, or the Lego Site) then that way he has to build the stuff himself, thus using more of his imagination, but the important part is: be there with him, help him build the stuff. If he is into challenges, have a contest to see who can make the biggest, coolest spaceship (I personally would lose on purpose, making him feel better, that and I am a Lego addict, so I can pretty much make anything from those little blocks.)
Another option: If you goto the Lego website, there is a program you can download that will let him make his own lego creation, then buy it, instruction and all. That might be an awesome thing too. (The program is a bit hard to understand at first, but if you help him, you both can totally figure it out!)
Onto my previous rant, you could even get him a DS, and your self one too, that way you both can play games, and he has a DS. Me and my mom love that (I know that is weird, I am 24 and my mother is 62) but we still bound over a good game of Nintendogs, or Millionheir.
I guess it all comes down to finding something you both will enjoy, and he will remember it, I don’t think there is a single person here on Fluther that would disagree. Sure, he might not remember it actively for many decades but he will. It wasn’t until a few days ago I remembered the K’Nex Ferris Wheel. My mom was going through old toys for a few friends of mine, and found the instructions for the Ferris Wheel, which got her thinking, and thus, me.