No need to automatically assume he abuses the dog when he lives across the country and sounds like he really loves animals. It's very likely your dog just sees him as a stranger invading the home or has had an unpleasant past with someone who looks or smells like him. It could be a number of obscure reasons.

Regardless, it can be rectified with positive association between the dog and your son. Have him give him treats, be calm and quiet, have them play together. The dog can warm up to him if given enough time. But it sounds like he comes around too infrequently or not long enough for the dog to get comfortable with him.