When my kids have wanted this, I've noticed that it feels special when they have things that look as much like the older kids' school stuff. Giving him the same kind of notebook/binder or whatever can go a long way.
Tracing and maze books, or pages that you print are fun. If you can put the pages in page protectors they can be used again and again with dry erase markers.
My little ones have liked to do the history pages with their siblings too. I even print maps for them if they want.
Other things that have been popular here are pattern blocks with pattern pages, a bin of beans with scoops etc, magnets (ABC, animals etc), painting with water, playdough, any kind of fine motor lacing etc....