I'm curious, is this dog going after your son's other dogs' too, or just yours and your daughter's other dogs? If that is the case, I hope he moves out soon, and the problem will be solved.
If he is doing this to everyone, well it doesn't surprise me. In fact, throwing dogs from three different families together and I'm surprised more fights haven't broken out between them all trying to vy for leadership of the group. Another reason he may be doing this is he is now an adult (at one year old) and is wanting to show everyone who is boss.
I would suggest what is often suggested in these cases. Start the dog on NILF right away so it can learn who is the real alpha in the group. Basic obedience may help as well. He is a young dog, so hopefully he can be rehabilitated. But he does need alot of work, which I hope your son is up to.
Not to be negative here, but b/c you don't know the dog's genetic backround, this may be occuring now b/c the dog is an adult and mature. Unfortunately, I know of a couple of rescue pits that when they reached adulthood (b/t 1-1.5 yrs), could no longer be trusted around other dogs. I'm hoping this is not the case with your son's dog, but it is something to keep in mind.