Yes, maybe your Son had a mild 'encounter' with the dog, and it left a lasting shock, and I don't doubt the shock would have carried into bad dreams, which to a five year old, could be just as real. Your Son is wise to be cautious of the dog, the dog could cause some pretty nasty damage, but given that he's known for being friendly, then I wouldn't discourage playing with each other, so long as it's 100% supervised.
Oh wow! Thank you for that response. I never thought about that and I bet you are right. I have been so focused on the fact that she is so gentle that I never even considered that maybe he had been too rough with her and she responded to him with a playful nip or something - I mean she weighs 85 lbs - shes a big dog - and maybe he just saw her in a different light. I will ask him about that tomorrow. Thank you so much for your perspective on that.
I think something did happen with the dog that he is afraid to tell you about. Not knowing your son, that's just my opinion. Even the most gentle, tolerant dog will snap or growl when scared or provoked enough, and if indeed they "pull on her" and "ride her" in play, he may have overstepped the boundary with her when he wasn't supervised. Also, she may not have been meaning to be defensive at all, she may have nipped him in play if he was rough, just like dogs nip each other. He has reached an age where in her mind he is no longer a baby to be tolerated like a puppy, and she is no longer young she has reached middle age and may be getting weary of the rough housing.