PS - it is easy to resent their 'part-time' parenting - but would you really want to be the drop in parent?
At the risk of soundig preachy, you are actually very blessed to be the one that gets to see your son's face on his birthday, and to know that you chose things he would like.
We can see things as a burden or a privilege. An example of this was my mother's final illness. This lasted a year. Going to see her involved a long and tiring journey three times a week. And I resented it, until I came to see it as a privilege - that she was being well cared for, that she was having the chance to be reconciled with her death, and that I had a comfortable car to make the journey in, and a family and colleagues who supported me . . and so on.
This is about YOU, and your responses. Not what your h is 'getting away with' That is his problem, ad I mean it seriously, and not in a flippant way. If he does grow up he will regret his behaviour, and if he chooses not to grow up he will lose everything that matters.
Sorry for the Monday morning sermon, but I love and like you too well not to point this out. Hugs, A