My Son's dog, a Mastiff/Pit/Rott combo just attacked our12 year old toy poodle.?

The dog probably wasn't socialized when it was a puppy that's probably why it is aggressive towards other dogs. If you don't want to put him down because he hasn't hurt a human then take him to obedient cles immediately before it gets worse and he ends up hurting a human. If he does injure a person or seems to then unfortunately put him down. But if he wants to keep him he can't have any other pets in the house and when he goes out for walks muzzle him just in case