If your husband gets a lump sum payment of Social Security that is owed to him for past years, it will normally be treated as just more Social Security income this year. If you have enough other income to make his Social Security taxable, the amounts for this year and the past years will just be added together and all treated the same. That could leave you with a big tax bill, so you would want to have some tax withheld from the payments to cover it.
There is an alternative calculation for lump-sum Social Security payments, called the "lump-sum election," that can reduce the tax somewhat in some cases. It basically calculates how much of the past-year amounts would have been taxable if you had received them in the earlier years. Then that amount is used as the taxable amount this year. TurboTax can do the calculations for the lump-sum election and determine whether you benefit from it, but you need some figures from your tax returns for the earlier years. Whether the lump-sum election will save you some money depends on your other income this year and in the earlier years. It's not likely to make a dramatic difference unless your income was substantially lower in one or more of the earlier years. The only way to know for sure how much it would save you is to do the calculations. Tax software for 2021 is not available yet, but you could get a rough idea of how it would work out by entering the information into a dummy tax return in the CD/Download TurboTax software for 2020.
If you have other income, it can be fairly complicated to figure out how much the tax will be on the Social Security, especially if you use the lump-sum election, and to decide how you want to cover that tax with additional withholding or estimated tax payments. You might want to seek professional help, at least for this year. (Most tax professionals would have time available at this time of year.)
Whatever you do about withholding for 2021, you will probably want to change it for 2022 because you won't have the lump-sum payments for past years. So be sure to recalculate your tax and withholding at the beginning of next year.
