I will be receiving $45000. From my ex's 401 once the divorce is settled. I receive soc sec disability and can not afford 20 percent taxes on $45000. Can he be required to pay the 20 percent

No. He can't be requird to pay it. And you may be able to roll it over to a qualifying account and not pay the taxes until its withdrawn. If your cashing it in as a windfall then you just get 20% less and pay the IRS.