Instead of a commercial retail product, have you asked your hair stylist for their suggestion? Hair dressers and barbers have access to professional grade products that main-stream comsumers don't have access to or don't consider/know about. Your hairstylist might be willing to get the razor for you with their beautician's license through a beautician's supply store.
Otherwise, perhaps you want to consider getting your son lazer hair removal sessions. Supposedly, hair can be permanently removed or signaficantly reduced in thickness. Haven't done it myself, so I don't have first hand knowledge..