Distract him, don't say anything negative about dad (even if it grates), and let him know you are never going to not be there for him, and do not feel responsible - the best way to cope with dad's who let their kids down is to let it wash over you emotionally.
What about baking a chocolate cake with him and letting him lick the bowl, take him to feed the ducks/scoot/cycle/playground, making a den, watching a children's movie/cinema, cafe with hot chocolate? Really anything where he feels he is getting attention from you? Get a friend/family member to visit?
Really nice advice above. I am sure once you get into your day he will be fine hun. Makes me sick that if it were you not able to facilitate the contact the courts would be on your back. This is not your fault no matter what has happened. Please make sure you keep a diary of non contact for any future court action, I am sure you are anyway. Have a great day tomorrow - you are all alive, well and together.
Can you get him to do some colouring pictures of how he feels today. You could create a special folder for him to have to put his pictures in.
Or you could get his favourite teddy and the two of you could create little stories of teddies adventures, ie teddy gets new shoes for big school, or teddies friends can't come to play and get him to tell you the story with you asking so what does teddy do now?
I did both with mine. It really helped just to get feelings out there.
My sons are 5 and 1 so it will only affect my eldest really. He know his dad isn't seeing him for his usual contact time and that he has decided to do something with his older children instead. This is court agreed contact and only for 5 hours each day. He has his older children on the opposite weekends from friday through to monday. I know my son is sad but he is very quiet about it and never says a bad word against daddy. This is all to get at me so i'm feeling very upset about it and responsible I guess. I've told him i'm sorry daddy has done this, that he is a wonderful little boy and we will do some fun things over the weekend. It doesn't help that the car broke down today so we can't go anywhere. How do I help him without making it worse or putting thoughts in his head?