Welcome, Chico.

One of the things I would have the vet check out is a brain scan. It is possible that a brain tumor could be the cause of this aggression, I have seen it in a few dogs before. One of the sweetest dogs I have ever known suddenly became aggressive and I suspect this was the reason. I never heard what they ended up doing with the dog, but the owner stopped bringing him to the PetsMart where I worked when it became a problem. Before that, he was funny and very easy going.

If this dog grew up with your son, it should have accepted him a long time ago. Admittedly, your son was less mobile until recently, but, none-the-less, should have gotten the idea before this that this is not just another "dog" of the pack, but the Alpha's favorite. I would also suggest obedience classes for the dog. It may well be that not everyone understands the "rules" of the pack and obedience classes would help establish rules for the dogs. It wouldn't hurt the other one to go, either. She may be very soft, but she might gain more confidence from classes. At least, that has generally been my experience.

You don't mention what mix the aggressor is? It may even be that the dog is so interbred that he will never have a decent personality and might be better off put down. I don't usually advocate that, but if all the above suggestions don't help, it might be safer for all to do this. Does the dog have any aggressive tendencies toward you or any other adult? Toward the other dog? Are there other animals in the house? Do you have crates for the dogs? Perhaps some time-out when aggression is exhibited would be helpful.

I wish you a great deal of luck on this and hope the dog doesn't have to be put down. Re-homing may be a possibility, but you have to be sure the animal won't transfer the aggression to someone else, as that could lead to some real problems for you. Well, see what the vet says and think about obedience work.