P.B.
I do not know if there is anything legal that you can do about it or not. You might need to touch base with a lawyer and ask. I just wanted to tell you that my X was horrible about getting the kids. I would have to drive the kids too him and provide food on the rare occasions that they went to spent the night with him.
My only suggestion is that you stress to him the importance for a regular schedule for the kids sake. They will be getting invites to birthday parties and to other activites and it is only fair to them to know if they can or can not go in advance. My kids were almost made to feel guilty if they choses to go to a party then to see their dad. It should not be that way. What about when you kids are in sports and things. They really need consistency.
Try to keep it about the kids - not about you or him. Try not to cut him up directly - but just focus on when the kids really need and want.
Good luck to you.