Dead beat / weed addict

What would you guys do? I’m looking for advice here. I have three kids with my fiancé. Baby #4 on the way. Tonight we promised the kids we would decorate the tree. They have been so excited to do it. Instead, he would rather go out and smoke weed and play mortal kombat with his friends. He literally just texted me “sorry baby this game is intense”. At this point I feel like he is a literal 16 year old child and is not ready for a family. I made a promise to my kids and I will keep that promise. I am taking the tree out without him.

Also, I have to mention he does this often. Goes out to “smoke” weed with friends because I do not allow smoking in my home. This is an every day occurrence. He could not take ONE DAY and stay home with us to do the tree. I know he is addicted to marijuana, and he keeps saying he will quit before baby #4 arrives but I highly doubt it. Any advice? 😕