Dogs play a lot rougher than humans, especially large dogs. Just as an example, one day my German Shepherd and her pal, a mixed breed collie were tearing across the park, each holding opposite ends of a large stick. Brandy abruptly changed directions, and Tasha did a complete somersault, and the stick was wrenched from her mouth. I started towards her, sure she must be hurt, but within an eyeblink she was back on her feet, and resumed the chase.
When I called her to me, I found her mouth was torn and bleeding from the stick!
Dogs will let people know if the play is too rough or frightening to them. They will growl (they growl in play, but you can tell by the body language if the growl is in play or for real) or just go away and hide. If the dog comes back for more, he's all right with what your son is doing.
HOWEVER, I own a book called "Childproofing Your Dog" by a well know trainer. In it he states: NEVER allow your dog to play unsupervised with your kids, no matter HOW well-trained it is. The unexpected can always happen. The kid accidentally does something to the dog that really hurts, and the dog retaliates. Those teeth are really sharp, and it only takes a few seconds for them to cause major damage. Better safe than sorry!
Edited to add: just remembered the author of that book. His name is Brian Killcommon (not sure of spelling
, though, and my books are packed away for a move...)