Starting a conversation

How should I bring up a conversation with my bf. We just got back together, I left him a month ago because he kept taking off to go drink. (We have a 2 year old & a newborn)

I just want to know how serious he is about this relationship, my trust issues with him leaving the house (he took off when he’d tell me he was just going to go get smokes, never came back), I wanna know if he’s talking to other people. We talked about this prior to getting back together, I just need more reassurance

I just don’t know how to start the convo lol.