How does child support work?

My fiancé found out his ex had their baby and she's been keeping it a secret (there's zero question to paternity), and now they're going to court to establish a schedule and child support.

My question is how does he pay child support if we are broke? We just had a baby and moved into an apartment so things are tight. Like there's no wiggle room- once the bills are paid and groceries are bought, we hardly have money left.

Will he have to pay back child support from the past 3 years even though he was unaware of the child's existence?

Id like to have an idea of how this may work.