"can florida state keep my fiancee's entire tax return for child support arrears for his ex wife even though we have a 3 year old son together and he is our only means of support?"

Yes of course.
What if he leaves you and your child to go have another; does he then have the right to stop supporting your child? It can happen, it happens all the time, and the government steps in to stop it. And you don't even have a marriage certificate to help you.

And why is he your only means of support? Where's your job, girl?

Of course they can take his tax return if he owes back support! Why isn't he CURRENT with his child support to begin with?

He has to support ALL of his children. Just because he's not with their mommy anymore doesn't mean that the children do not still need his support.

And if he can't afford it, then he certainly shouldn't be having MORE kids with YOU. He should be supporting the kids he has FIRST.

"can florida state keep my fiancee's entire tax return for child support arrears for his ex wife even though we have a 3 year old son together and he is our only means of support?"

Florida doesn't have an income tax. So what tax "return" (i.e.: refund) would this be?

To agree with everyone else... YES. Tax refund money owed to him by the federal government can be intercepted to offset a child support obligation. Generally, I would say to file an injured spouse claim to get back the portion of the refund that you are entitled to but you have already stated that you are not working so none of it is yours. In the future, if you do begin to contribute income to the household, you will want to file an injured spouse form along with your taxes to receive the portion of the refund that you are entitled to.