Why is he doing this?
I used to ask myself the same questions about my sister about so many of her actions. I spent a lot of time trying to figure out her motivations. I still do have empathy for her, but I realized one thing: I could spend years trying to figure out her motivations, but if SHE didn't want to figure things out for herself it wasn't going to matter one whit.
Please do consult with a lawyer. You need a child custody agreement with teeth to protect your children. What happens if he shows up drunk for a pickup? What happens if he refuses to give a time for a pickup (apparently my sister does this all the time with her ex) and you burn through an entire day so you can be available? What are the consequences if he fails to pick up his kids? Will you let him schedule/re-schedule whenever he feels like it? What about first refusal? And what about SoberLink? There are just so many details that an experienced lawyer can guide you through that you just wouldn't be able to think of.