First ejrjwk, he must want help. Bipolar disorder in itself is difficult to say the least, but you throw drugs and alcohol into the mix and you have a recipe for disaster.
You can tell him you love him, that you're there for him if he needs to talk but he needs to seek treatment if he wants you back, just my opinion, but you certainly don't want to expose your child to his lifestyle.
I have a friend who tried to be supportive to her husband who had drug and alcohol issues, (mental health ones as well it seemed) and while she did NOT participate in his activities, the state still came in and took all four of her children. Two years later they severed her rights and she didn't even get visitation. This is something she will regret all her life.
I know you love him, but make your child your priority. Nothing is more precious than that baby. I know it's his baby too but being able to create a child doesn't necessarily make one father material. This is certainly difficult for you, and I for one know first hand how frightening a prospect single parenthood can seem. But sometimes it ends up being the easiest and most beneficial route.
Hang in there hon.
Keep posting, we're here for you