Erector set has tons of individual projects in one kit, some motorized. The nuts and bolts are a bit small, but once the handling is down, projects go smoothly. I'd hazard to guess there are many online resources for additional things to build.
Scratch for programming can fill the same brain space; also videos of different progams to do. The nice thing with programming, is he can explore a few variables here and there to get a sense of experimenting without the same sort of load Legos or trying to freeform with an Erector set does.
My DS was mixed on Snap circuits, really liked them around 4 or 5.
On Amazon search for Erector Super Construction Set 25 models
