Hi A.,
First off all, I just want to say OF course you want to put your son ahead of your dog. It was silly for anyone to say different.
I have dogs and almost everyone in my my family has dogs so I really feel for you. One of my dogs showed aggression towards kids until our daughter was born now she just mostly stays aways from them,except for our daughter, she loves her up. So we were very lucky.
But here is another story. When my daughter was almost a year old we went and stayed with my husbands brother for Christmas. Long story short their dog bit her as she crawled by him. Luckily he bearly broke the skin and just missed her eye. So she had what looked like a few scratches and her eye was a little bloodshot for a few days.
My Sister-in-law made all sorts of excuses for her dog...he'd had never done it before, he wasn't feeling good and she also had the nerve to say my daughter crawed onto him. Which she didn't my Husband was right there and my Siter-in-law wasn't. Needless to say we left and went and stayed somewhere else for the rest of the week.
Less than a year later they had a son. And guess what, the dog did the same thing to him, again on the head. My husbands Brother was ready to bring him out back and shoot him. But my sister-in-law made more excuses. They still have the dog. Supposedly they just keep him in another room. Thats not really any sort of life for the dog either. I don't think I will EVER be able to have any respect for my Sister-in-law again. Luckily we live 1200 miles apart and we actually haven't seen them since. I don't know if I will be able to not say anything to her. I have been in such fear for my Nephew that I have almost called the police in her town and just asked what to do. He is a big dog and I have no doubts that he could cause serious damage. She is clouded by the fact that the dog was her baby for 8 years before she had her son. I think she is a terrible person for it. It has caused kind of a huge rift.
I know this isn't your situation exactly but I wanted you to know, what ever you decide to do, keep in mind that the people in your family will have their own thoughts and feelings about it too. Like I said we were lucky and I couldn't imagine getting rid of one of my dogs but I would in a heartbeat if they had ever showed any aggression towards my daughter. It is easy for me to say though because I have seen what can happen.
Also if it makes you feel any better I have a cousin who had two rescue dogs. He put lots of time in "puppy classes" and training because they were strays. He loved both dogs a lot but when his son was born, one of the dogs showed aggression. He kept working with it but it didn't really help and he had to bring it back to the shelter. He felt aweful but not as bad as he would have felt had something happened to his son.
Wow this got LONG sorry. I just really feel for you. This is such a personal choice. No matter what you choose it isn't fun.
Good Luck!
~A.