Hey Thinking about you....looking for clarification....what is the arraignment for his time with S? Does he take him the standard one day a week and every other weekend? Now, for your S there needs to be a schedule that he adheres to....you S should be as important now than he ever was. This is a tough time for him to and everyone needs to think about what's best for him.
If you and H schedule something, like family night....and he doesn't come through, you go anyways. And you make it clear that YOU invited HIM and even if he didn't show up, you are still going to go and enjoy yourself, with or without him. Let him know, he is NOT the end all be all and YOU do have a life outside of him that you will and can live.