You can file a request for orders to modify this term. Or, since this has to be by agreement you can tell him that you cannot agree since he has failed to keep this additional time.
Is this a contract or an actual court order? If it is an order, you can file a request to modify the order. You should request an actual schedule be in place rather than an order which is so vague and easier to back out of. Unfortunately modifying the order does not mean he will not cancel on you still in the future.
I agree with the other two attorneys answers here. I strongly recommend, though, that you at least consult with a lawyer who can look at your documents and give you specific advice based on the history of your case. In the meantime, I suggest that you not tell the children that dad is coming in advance. Reading between the lines, it seems like he wants financial credit for his overnights, but he does not really want to parent the kids. That is a setup for disappointment now and into the future. Do your best to manage your kids' expectations and make it clear as best you can that it is not their fault that dad does not want to spend time with them.