K.K.
A dog's breed has no effect on whether or not it can or will bite- that's why I say to people who hate pit bulls that they are wrong, depending on who owns the dog. How a dog is depends on how you raise it to be and I am sure you haven't raised your dog to be violent or aggressive.
That being said, any dog can and will bite or become aggressive or annoyed if it is instigated. Being that your son is 7 months old, he obviously wasn't trying to annoy the dog. The dog, however, doesn't understand that. I'm sure the dog is a little jealous of your new son getting attention, and also doesn't understand that if he reaches for him or pulls on him in any way, that he isn't threatening him.
I don't know if I would keep the dog or not honestly, because I'd be afraid that I couldn't train the dog well enough so it wouldn't happen again. BUT if you are going to keep the dog, I would be sure to talk to a dog trainer about how to make sure it doesn't happen again. Obviously, never EVER leave the dog alone with your son. I would make sure that if your son is out and about crawling on the floor, to keep the dog in another room until your son is old enough to know he can't pull on the dogs ears, etc. When your son gets old enough, you have to teach him "nice" and how to approach a dog the right way. I would read up online about dogs and small chldren to reasearch what you should do to better familiarize your dog with how to behave around a small child.