S.T.
it's time to be firm with your little boy and stop trying a bunch of different things. he's not getting it, and it's not fair to the dog. i'd fix this young hooligan with my scariest mommy eye and tell him, 'if i see you being rough with this dog one more time you are to spend the rest of the day in your room' (or whatever your consequence for bad behavior is.) and enforce it, relentlessly.
don't ever let the dog be alone with him. ever.
no long explanations, no justifications or lectures or rationalizations or applications of logic. lower the boom swiftly and hard each time you see even a slight infraction. do it until he gets it.
if you can't get this under control, rehome your dog.
good luck!
khairete
S.