For me the 40 hours of community involvement was easy. The school is a great place to help out at. Some things I did was help out with the craft show, with the reunion, with a sports team, sorting library books, registration, open house and painting a mural. If your son likes sports it might be good idea to help out with one (doing score keeping, marking the lines for football or keeping track of the time).
If your looking at an easy way to complete the 40 hours is to help sort toys for the Chum City Christmas Wish. The center for sorting the toys is in Mississauga I believe and it was a lot of fun. All you have to do is sign up in advance and when I was there they had people helping as young as 9. One or two day events are usually the best bet to get it done (so you don't feel bad leaving and I found the screening for longer term volunteer jobs (salvation army) was a lot harder).
Don't know if this helps but it might give some ideas on what to do for the forty hours.