M.L.
The trouble is that if your son plays too roughly with the dog and scares her or injures her, she could begin biting defensively, and then she (not he) will be really out of luck. If your son's best friend were a person, that person could get up and go home. Your dog won't have a home.
Your little boy might be considering her more a toy than a living thing.
Consider that you have two children (at least) - one four-footed and one two-footed. You will HAVE to supervise them when they play. It might also help to play with the dog yourself, and to take her for walks, as a way to model the right behavior.
Keep on showing your son how to treat her gently and how to play with her properly. *Expect* him to follow your lead. He just may not be listening to you!
Talking to your vet about this might give you some more help. Vets love to educate children (and adults) about the right ways to love animals.
I'm so glad you care about this.