I suspect that most dads resent child support. Having to pay a certain amount at a certain time takes away their ability to have control. Some would be OK with paying if it were there idea about amount and time. They would be responsible about it too. But I'm afraid that many divorced men do not have a sense of responsibility, perhaps because they do not really understand how much it costs to raise a child.
How long have you been divorced. My grandchild's father complained for the first couple of years but it's been 5+ years and the complaining is not so much. I think it did help for my daughter and perhaps his mother talking specifics about cost with him.
For example, he said my daughter would have to pay rent even if she didn't have their son. Good point. But she does have his son and he does have to live somewhere. It's not only the mother's fault that the marriage didn't last. She has the lion's share of the work. His way of sharing responsibility for the creation of his son is to help with the rent. Another way of looking at it is that the woman may be able to find a smaller place that costs less if she didn't have a child.
My granddaughter began letting her ex know the cost of things as related to their son. Eventually he seemed to understand better. Eventually he and a girl friend moved in together. Up until then he was living with his parents. I think getting a life for himself also helped to reduce the animosity. Living on his own, which he'd never done before his marriage to my daughter, helped him to realize how expensive living is.
He'd complained about money when they were married, too. When my daughter was off work on maternity leave, he thought he should have total say over how the money was spent because he earned the money. My sense is that many men have not had realistic experience with money and expenses as well as how to share resources. Their mothers took care of them and they expect their wives to do the same. Cost accounting may help in reducing the amount of animosity they feel at having no control over their income.
It is always easier to blame someone else for one's own money problems. I hesitate but I feel that I must respond to the idea that because a father makes very little money he should not have to pay the basic amount that the courts order. Making this baby was his choice. It costs the same to take care of a baby no matter how much money the parents have. The chart that determines how much child support to levy takes into consideration both parent's income. When there is not much money the child still needs to be taken care of. If the parents had remained together they would have to find a way to make ends meet. They still have that responsibility when they're apart. Because they cannot cooperatively resolve the issue for themselves the court must do so. The court is very limited in what they can do. The system is set up to consider basic circumstances but it is not able to include personal circumstances other than the amount of money available at the time. A levy of a flat amount is all that they can do. The couple themselves have the right to change their own circumstances, such as getting a better paying job, but usually they are too involved in emotional issues that they aren't able to manage other changes or ways of cooperating. Or it may be that a better job can't be had. It's still not the other ex-spouses fault.
As to whether or not the mother is spending money on herself, rather than on the children. If the children are fed and clothed, live in reasonable circumstances, how can one know that they are not spending the money on the children? She has other income or she couldn't spend the money on herself and not the children. Once divorced, one has no control over how the ex spends her money as long as the children are taken care of in a reasonable manner. I see the accusation that the mother is spending the money on herself and not the kids as most often a continuation of anger because the father has lost control of what he sees as his money. The father and mother share equal responsibility for the welfare of their children. Yes, the father is at a disadvantage financially because he does have to maintain a separate residence for himself and the mother doesn't. But the mother "pays" for having help paying for her residence by having most of the responsibility for physically taking care of the children. As we all know being the live in parent is a difficult role.
I suggest that it may help if you can show your ex some sympathy for his lack of money. And at the same time give him receipts or in some way account for some of the expenses. Showing him a budget might help. But it will only help if he wants to share responsibility. It will not help if he needs a reason to complain. Some people are not happy, literally, unless they can blame others for the circumstances that they help create.
I do wish that young people were aware on a deep personal level of the responsibilities, to include the expenses, of raising a child. It's not too late to learn this after the fact so that they can accept the responsibilities. It's just a matter of finding a way to show them that has meaning to them and in a way that does not feel critical of them.