Depending on how into it he is- you may want this-
http://www.middleschoolchemistry.com/

Or Lego Chemical Reactions is really fun- designed for a class, but you can manipulate it smaller- http://howtosmile.org/record/5478

If you don't want a whole curriculum, here are a bunch of experiments and projects

http://howtosmile.org/resourcesearch?q=resourcesearch&ddssearch_q=chemical&verb=Search&search-type=activities&ddssearch_n=10&ddssearch_s=0&%2FsmileItem%2FactivityBasics%2FageRanges%2FageRange%5B0%5D=8-11+years+old+%28grades+3-5%29