C.S.
http://www.amazon.com/Super-Childrens-Vinyl-Coated-919-53...
This is the life jacket we use for our son...he is 3 1/2 now and we have been using for some time now(I think we started using it when he was 1 1/2). He loves it and he feels so safe in it. It is cost guard approved. It comes in different sizes. The one I have is x-small for user weight 30-50 pounds. I am not sure if it can be used for your son. It has a strap that goes between the legs so the vest does not ride up. I found mine at American sales in Bridgeview but I am sure you can call around.
good luck