husband dog snapped at toddler, won't get rid of him

My husband has a dog from his first relationship. The dog is 6ish and was always a good dog until our son was born. Every time our son was near him he would softly growl. I was mad but my husband insisted that it was fine and would work out in time. Well today my 20 month walked past the dog and completely unprovoked the dog tried to bite my son really hard on the arm and I just happened to be right there and somehow managed to grab him in time. I lost it. My husband told me its no big deal and when I told him I want the dog gone he lost it and is pint blank refusing to get rid of the dog. I'm very seriously considering taking my 20 month old and 4 month old and leaving. I refuse to wait until it's too late for him to get some sense. Am I doing the right thing?