Who does this with a 4 month old baby at home?? 
The thing is, if he did go out regularly and had a nice, reliable group of friends, had a few drinks then came home at a reasonable time, not too drunk ,then that would be fine compared to a couple of massive, ridiculous, potentially dangerous benders every year, involving drugs, with alcoholic relatives!
It seems odd he doesn't have friends but gets like this with his own family.It sounds to me like some of his family have serious issues.
I don't know any married man with a baby who would do this and not even give you a one sentence text to reassure you they are okay but staying over somewhere. He has a family!!
The fact you have given him an ultimatum last time and he has done it again - I don't blame you for packing a bag. You need some time apart for you to think. He clearly thinks he can just walk back in, mumble a few apologies and go back to normal as if nothing has happened.
If you do stay together I would want him to go to Al anon and narcotic meetings / have counselling and never drink or take drugs again. Because he cannot be trusted to do the right thing when he does go out, and even if he says he isn't an addict, clearly something odd is going on as he hasn't the maturity to know when to stop and come home.
Who the hell is he out with anyway? I would be ringing them.