Stomp rockets were big hit around here at that age.
What about sports or outdoor equipment? Rollerblades, scooters, baseball gloves, nerf football/basketball, stilts
There are a few science kits that he could do that would be fun. Make sure they're age appropriate though, as some of them really are geared at older kids, and it's not much fun if they can't do them themselves.
Board games are a good choice: My son liked :Cranium Cadoo, Sorry, Monopoly Jr., and some of the Family Pastime games (sorry can't remember which). It's a good age to introduce checkers or chess if he might be interested. (Our son wasn't.)
Would he be interested in a marble run?
Kids this age often like 'projects' they can do -- building a bird house, painting a plate, weaving, etc.