How to determine whether to file injured spouse form or not?

My husband owes approx $70K in back child support and approx $6k to the IRS. He has been marked as uncollectable status by the IRS but I am sure they will still take his return and if they didn't then child support will. He made approx $51k paying $9000 in federal taxes ($15,000 plus was paid to child support) and I made approx $21k w/approx $1700 in federal taxes. We have 1 child together and I have 2 of my own. Child support says he has to claim our child we have together or he will need to pay more child support per/mo. I thought I read somewhere that in ordered for the injured spouse to benefit the injured person would have had to had the bigger yearly gross income. We have been married for 6 yrs now and I have never filed this because I didn't know if there is any benefit to me. I have always file MFS. I am always told how I am losing out on so much money due to the EIC credit we miss. The tax preparer we used last year said he looked at it and it wouldn't have helped to file MFJ w/inured spouse form. I am ok to wait longer for my refund if it means a larger refund than usual. Can someone please give me advise on this?