Child support

I've been fighting my ex for years for support. My son has a disability and gets ssi and I have him full time, his dad takes him on Saturday nights. He's never paid me anything for him. Never helped with clothes, nothing. I took him to court for support, he threw a fit when they suggested he pay $480 a month, so I said how about $200? He lost his mind and told me I don't deserve it and that if I need financial help that he will take his son then.
Has anyone dealt with this with their ex? Can he take my son from me for asking for support? 
And the big one, he pays me $20 a week for him now. Which is not much. How much support does your ex pay you for your child?? What's a "normal amount"? (I know it's income based)