This is a link to a website that has instructions for building every Lego set ever made. Just select a year at the top and then click any set that looks interesting. You may not have all the pieces required for many of them but with enough time spent browsing (it could take a while just to decide on one), I'm sure you can come up with something that you and your son will enjoy building together.
http://www.hccamsterdam.nl/brickfactory/year/index.htm
