SSDI and Child Support?

My daughter has been receiving dependent benefits since she was about 2 mos. old. Her dad continued to work for himself earning unreported income while on SSDI and, before getting involved with another woman in 2012, he occasionally helped me pay some bills when I got very behind on them. He paid no more out of pocket to me while involved with this person and even lied about his income when I would ask for help paying anything.

In October of 2012 approximately, he got involved in the 1st relationship since we split up in 2008. Within 1 year he got involved in 2 different relationships with women, moved 3 times (in/out of his apt into 1st woman's home, then back into his apt, then out of his apt. and into a home he and his current GF were renting together, and had another baby (with current GF).

He promised that they were not going to file for SSDI dependent pay for this child they were expecting since he wasn't paying any ordered child support for our daughter and he knew we struggled financially even with the monthly dependent pay she got from SS.

He has a total of 5 children now, three are 29 years old or older from 2 previous marriages, our 7 y.o and his now 1 y.o daughter with his current GF. SS sent me a letter notifying me that he filed a claim for dependent's benefits on her behalf three months ago and that they would be cutting our daughter's monthly amount by 50%, as this other child has entitlement to half the total dependent benefits amount, and that our daughter was overpaid for 6 mos. (since they go back retroactively for 6 mos. prior to the date of filing if the child was born and respectively entitled to pay at that time) and that SS is going to pay her 6 mos. retroactive. His daughter has a legal right to the benefit; although, I am not happy for my daughter who has been getting 100% since she was 2 mos. old, and for which reason that I never filed for a court order for child support from him to pay for her as I am the custodial parent.

He states that he isn't working anymore since he takes care of their daughter during the week so they live on only his SSDI and paychecks from his GF who works full time. I would like to know if he could be made to pay me child support for at least the amount that our daughter is now losing from her monthly dependent's benefit. This would be approximately $300/ month. Keep in mind I get public assistance, and have had both Medicaid for her and EBT benefits since she was born. His and her daughter's birth and prenatal care like mine were all paid for and covered by Medicaid as I was unemployed and without insurance. She, although insured, was not covered for pregnancy healthcare benefits. He claimed disability and being on SSDI as the reason he couldn't be forced to cover the cost to have our medical care and birth of our kid.

How is child support calculated and does the formula for it vary by state? He chose to have another child without regard to the one we have together. Since his baby was born he no longer sees her on a weekly basis to do things together, he gets every other weekend, overnight Friday and Saturday per our visitation order, but rarely has her for more than one night and ends up calling me because she wants to come home, and brings her home; or he has something else planned during his visitation weekend. Never does he allow her over if she is ill, or he and his GF and/or their baby are ill. His relationship with his daughter has taken back seat since he had another child, and he now stays at home to care and watch his baby so his GF can work (which is the very thing he told me I should not be doing, but I was out working full time, or two part time jobs, and while going to school. He claimed he knew many female friends with children who had no problem juggling work, school, and single parenting).

I know his views and choices are not important when it comes to his financial responsibility to our daughter; but it does show an string of irresponsible choices he has made regarding having two children for which he took no financial responsibility for their births, nor paid any funds for monthly support or covered costs for medical/dental coverage on our daughter since her birth. He has a biased judgement about my parenting and ability to find and secure full time work and attend school. Apparently he sees nothing wrong with giving up working to care for their daughter, and isn't having to deal with school in order to secure a better job/new career either. All the time he has free, and he does not even try to see our daughter more. I guess we should be grateful that he comes to have lunch 1-2 times per month with her at her school and brings her McDonald's to eat. I am very sad for our daughter who is too young to understand things and often gets let down by her father who makes selfish choices and often spends very little time with her during his visitations which are never just him and her so she can get personal time with just her daddy.