Baby daddy problems ):

Anyways, I told him he needs to contribute monthly to the cost of the baby. The state of CA estimates his monthly cost at almost $1000 a month. But he only wants to pay $300 and won't put baby on his insurance plan because it costs to much (he's an asshole). I told him $500 a month and he can have the cat and I'll put her on my insurance. I really believe he should have to pay me the $500 and have her on his insurance. We purposely got pregnant, we both wanted a child and were very excited. Now he's seeing someone else and doesn't want to own up to his financial responsibilities.. I told him if we can't work it out between us that I'd have to seek legal action, he said he'd quit his job so he wouldn't have to pay me anything. And that he'd file for full custody because I unfortunately have to live with my grandma temporarily and she smokes outside and in the garage (he thinks the courts will grant him custody because she smokes cigarettes  outside the home). I don't know what to do....he makes way over $3000 a month after taxes, I make about $1600 after taxes and insurance...rent is $700 a month for me because I'm renting a single room from grandma...and rent in the area is roughly $1500 for a 1 bed apartment. He's being disgustingly unreasonable. 

Any advice??? Baby is only 2 weeks old and I'll be breastfeeding to 1 year if I'm able too.