I need to leave my husband

My husband is borderline alcoholic. He was sober when we got together and I didn't know he had a previous drinking problem until we had moved in together. 7 years later and he's been an alcohol for the last 6 years or so. He promised me he'd quit drinking when our son was born. Our son is now 8 months old and my husband is still drinking everyday. I'm currently sleeping on the sofa. I have given my husband an ultimatum but he keeps saying I'll never dollow through with leaving him and I think he's right. I love him so much, with all my being, he gave me my son! But I can't be the only one fighting for this marriage. I feel so broken and I can't fix me or him.