My boyfriend is on social security disability and has a back tax debt he gets less then $900 a month, can the IRS levy his income?

No, they cannot, but they WILL try. Your boyfriend needs to submit the necessary paperwork that shows he needs every dime of his Social Security payments to survive. This includes a statement of his net worth. If he has other assets, such as a paid-for house or investments, then the IRS is free to garnish the Social Security payments or to put a lien on those assets.