I agree with the reflections offered above.
I would look into my own responses rather than try to make him wrong or suspect. It does seem perfectly reasonable that a responsible parent would place his holiday time with his yound daughter as a priority, and, in fact, he did not even say "no" to your invite. He merely said he needed to double check his availability.
If it were me, and I were in his position, I would do the same.
Personally, I think the holidays are an emotional minefield, with all the expectations and pressures we allow ourselvees to operate under...and for what?
As part of my recovery, I reserve the right (and responsibility) to decline any invitation that doesn't have special meaningfulness to me.
Now, if your christmas party AND him being there with you at it mean so much to you, why did you plan it without consulting him as to the date? If you are going to use it as a litmus test of his commitment to your relationship, you should be willing to offer him the same commitment and welcome his input on the planning of the event. In other words, include him in everything, don't just spring the date on him and then expect that he will be able to be there in the way you need him to be. This will bring you two closer.
hope this helps.
He sounds like a good guy.