I would also say it wasn't a big deal because my FIL's dog will snap and bite, not breaking the skin EVER, if her space is invaded. She was a breed dog that lived in a cage, never really socialized to be around people much less kids.
This dog will take some time to get used to the new home and I would ask that they put up a baby gate or something to keep the dog in a smaller space until the dog gets better socialized to your son.
They need to spend some time together so they can get familiar with each other. He needs to get to know her signals that she is done and needs to be left alone.
One thing that your post points out is they didn't know your son had been bit. That tells you something. It wasn't hard enough to make him cry, he let it happen numerous times without saying anything, he might have been annoying the dog too, etc....
If he had been crying they would have noticed. If he told them the dog was chasing him and biting him they would have noticed and put a stop to it. If the dog had been growling and barking at your son to get him to go away they would have noticed.
The dog needs some time to get used to your son being there and it will take longer since he doesn't live there. Your son needs to recognize that the dog is not like the rest of the animals he's come into contact with. He needs this to be a learning experience on how to interact with strange animals and how to use his words if he's uncomfortable.
There is no telling what this little dog has been through. Sometimes they have been tortured by children, think of the little boy and how he treated toys in Toy Story 1, no one can tell how a new animal is going to react to a situation especially a rescue dog. They are usually sent to a shelter because the family didn't have any idea how to socialize or train this animal and they found themselves having a pet that didn't know how to "go" outside or how to get along with them, or some other area that the first family had no idea.
Just talk to ex and ask they help the dog to socialize with your son and to keep them separated a bit until they can see the dog is more comfortable with him.