I don't own a .32, but I've been looking at them as well for squirrel hunting. Here is the one that I've been looking at for a while now.
http://www.traditionsfirearms.com/es...Code=R26128101
How old is your son? Why is it he can't handle the 50 cal? Is the recoil too much, or does the stock not fit him properly?
You might do well to find an older, used muzzleloader and have the stock cut down to fit him properly. Just a thought, FWIW.........