If your ex does not want to pay for his son, then he is hardly a caring role model is he? Do not let him off the hook. You will find that men who say they cannot afford to pay for their own children (and that is neglect, make no bones about it) can often afford to pay for the everyday luxuries they feel they are entitled to be it alcohol, weed, tobacco, gambling etc. What about food, heating, clothing for your son?
I have been pressured to accept money and stay away from the CMS. I will reiterate, do not do this. They want you to stay away for a reason. I was offered £200 per month to keep it between us as a grandparent carer. I declined because I knew he would stop paying, it wasn't about getting more. It was about securing financial support for my grandson to which he is entitled by law.
When I went to the CMS, he claimed to earn £19,000 self employed. Their fraud team found it was £42,000. He wanted me to agree to direct pay which I foolishly did. He stopped paying. It is now collect and pay which attracts a 20% charge to him by CMS.
He has stopped paying completely now, but I am used to not receiving anything. CMS have applied to the courts for a liability order. He is not someone who cannot pay, he is someone who doesn't want to pay. I suspect your ex is the same.
The CMS will take his new family into account if he has more children. They set a minimum payment which is 12% of his gross income. Apply now because he will go through the same motions as my gs's dad and child support is backdated to the date of your application. Good luck.