If the pension income is paid to his estate after his death and his estate files a tax return, the tax rate will be whatever his rate is for his overall income and deductions. There is no special tax rate for income that will be used to pay a debt. Whatever is left over can be distributed to his debtors. Whether child support comes ahead of other debts he might have is a matter for state law. Any mandatory withholding on the pension is only an estimate of tax he might owe, the actual tax is determined on a tax return and depends on all his tax factors. If you have the estate collect the pension and pay tax using an estate tax return, be careful not to distribute all the cash from the pension unless you hold back enough to pay the tax, otherwise the tax can be clawed back from the recipients of the money. (For example, if the pension check is $100,000 and the broker holds 10% and the estate distributes the other $90,000; you will eventually find that the tax is actually about $22,000 and the estate or the IRS will need to claw back $12,000 from the heirs who got too much in the first place).
Expert help may be called for in this situation.
The other alternative would be that he died without paying his debts and the debts die with him. The pension gets distributed to the designated beneficiaries, or to his legal heirs if he did not designate a different beneficiary) and the beneficiaries pay tax on the amount they receive.
If the people to whom the child support is owed are the same as the pension beneficiaries, might be simpler to just close the estate and pay out the pension. If the child support is owed to different people, and you want to make sure that debt gets paid out of the pension before any other benefits are paid, you probably need to talk to an estate lawyer.
