:grouphug: Hard decision, but better for your boy and the dog in the long run. (Please be very honest when you return him and explain in detail your experience with the dog and your son.) The best home for the dog may be one without children.
When you are ready to look for another pet, consider scheduling over night or week long visits in your home with potential dogs. For young children, I would tell them we are pet sitting vs. seeing if a dog is a good fit. (Lots of kids really, really want a pet to stay and overlook or hide behaviors that should be noted.)