How should I respond to this?

Started on 2/10 but never posted

My ex wanted to talk to me tonight, I figured it was about the monster's summer trip. The monster has been saying he doesn't want to go and I figured he wanted to know if I had made any headway with the monster. Boy was I wrong, instead he had a favor to ask of me. He told me that he did his taxes and would be getting back a substantial amount of money since he claimed his girlfriends kid. He asked me if I would give him back some of it since he is only getting it bc he is claiming another child. The amount he asked for back is less than half of what I might be getting back, I will give him that much he didn't ask for it all back.

If you know me you know that I thought about this and am still thinking about it. But I did tell him No and really hope to be able to hold my ground with that answer. I am so taken back that he would ask me this but really why should I be, I am the one he called when he was upset about his mistress and the one he asked for rent money from, the one who helped him out when he was evicted, the one he called and complained about the second wife-now soon to be 2nd ex wife. I am way to nice to this man sometimes so he will of course ask me for help all the time.

I explained to him that if I were to give him back any part of the money I am doubly screwed bc not only will I be out the money but he will be off the hook with the state of MD and I will have no recourse bc he will have been given the credit for it. He doesn't get it. He said he would pay me back which I have heard so many times before I have lost count. I reminded him of the train ticket I bought for him and the plane tickets I bought, both of which were recent things I have paid for that he hasn't paid me back yet. I reminded him that I set up a special back account that he requested that he was supposed to put money in. I am sure he has lost the bank account.

He actually started attacking me and said since he pays his child support why doesn't the monster have stuff when he goes to visit his father? Like what I ask. I sent 10 pairs of socks, 10 pairs of underwear, 10-12 shirts, and all his shorts. What else was I supposed to send? The ex says the monsters shoes were falling apart. Really the ex bought the kid a $30 pair of shoes and he is complaining. When the ex was in town he asked if the three of us could go shopping for shoes for the monster and the two of them picked out $75 shoes that I paid for. Those were paid for when the ex wasn't working so he cannot say his child support went to any of that. I explained that child support also goes towards living expense, food and shelter. That the child wants to play sports this season which is all going to cost me and yet somehow the ex feels he has the right to get on my case.

The worse part of this whole situation is that the child was in the living room with me and heard my end of the conversation. I have tried so hard to keep this stuff from the child. I don't want him to think his dad doesn't want to help. I know that it's his father's mindset and that he and I see things differently, however the child doesn't understand that yet. I had to show the kid the child support website so he would see all of the payments the ex has made recently. I asked if he had any questions and he said no. However I could tell his mind was working and he was upset.

4/14 addition:

Since this phone call the monster has definitely displayed more hostility about his father. It has not been fun. I know the monster is old enough to make up his own mind about his dad I just didn't think he would be so vocal about it. I am not sure why I am so surprised I am pretty vocal in my opinions so it shouldn't be a surprise that my child is too. I was just hoping that he wouldn't be so angry with his father but from what I have heard from others and read about its not uncommon for children to be angry. I just hate to see the monster hurting.