Friend of mine paying child support for 2 children that aren't biologically his.

One of my best friends got his girlfriend pregnant in high school and never graduated because he had to work full time to support his new family. Over the next 4 years she got pregnant another 2 times, and now my friend has 3 children.

They've been separated for 3 or so years now, and last year he got paternity tests done which proved that the first child and the third child are not biologically his.

He is currently paying $500/mo in child support and is living rent free with another friend of ours because he can't afford his own place. He cannot afford a lawyer. He has a learning disability and is easily manipulated, but is trying to get out from this situation he's been stuck in for 8 years.

What are his options? The middle child is his, but the one she had at 17 isn't and neither is the youngest. She also had a fourth child long after their separation and she gave her his last name, is she allowed to do that? He hasn't accepted any responsibility for the fourth but is often left to babysit her when he has his time with "his" children.

Edit - My friend accepts responsibility for the three boys he’s been raising, he just needs help establishing a more equitable childcare plan.