My fiancée has back child support ordered on him while he was incarserated and he owes ten thousand dollars and we have two children to take care of so I wanted to know how much of his income tax they can garnish or will they garnish it all?

If he owes back child support, then ALL of his tax refund will be confiscated each year until the child support is paid off.

You can file an Injured Spouse Allocation (Form 8379) to keep your part of the refund safe, plus any Earned Income Credit you may have coming due to the presence of the children, but the portion of the refund attributed to HIS salary will be taken to settle the back child support.