If my boyfriend and I get married do I have to pay his child support if he didn't have a job and would the mother be able to take my tax return. If so is there some kind of prenup or other advice you could give me like keeping my last name.

There is a form called the Injured Spouse Allocation. It is submitted with the tax return to allocate your and your husband's income and thus split on a proportional basis the tax refund.

This prevents YOUR refund from being used to pay your husband's liabilities.

If he is not working he needs to go back to court to get child support modified to a lower amount. And while you don't have to pay it, he can be punished by state law for not paying

NO you don't have to pay it but if he doesn't his ex-wife can really put it to him. They are their kids not yours so you are not responsible for their care in any way.

Note that my advice applies to the FEDERAL tax return.

Most states have similar allocations, but vary as to HOW the allocations are applied.