My father left us when I was 12 (for another woman, he did not drink). It to this day is with me. But I had a strong Mom who did not cover for him. She never said a word bad about him (even when I found out as an adult that he never paid child support and left for a close family friend).
She let me form my opinion of him with his actions and words. When I was younger, I thought to myself "why doesn't she explain more of what is going on?". As an adult with children, I think I know why.
I know you are at the beginning of this journey with your husband, but it can end up ok, you have choices too! I'm sure that isn't the way you had it planned. The main reason I stopped drinking was due to the guilt - my wife did not marry me to have a drunk roommate!
I hope your son had a great birthday and the selfishness of his father did not dampen it.
Toss