Can a primary borrower sue the co-signer?

My husband (ex now) and I took out a Sallie Mae loan in my name with his father as the co-signer in 2012. His father co-signed under the condition that his son would pay it back-which is what my ex said he would do bc he’s was finishing his anesthesia residency and could afford to pay it back. Everyone agreed that it wasnt the fathers responsibility or mine. We got approved and got the loan for 20k. We split the loan down the middle. We got divorced the next year and when I asked him about the loan, he said, “it’s in your name, it’s your problem.” And his father agreed with his son disregarding the verbal agreement we all had. Meanwhile, my ex took half and spent it. Is there anything I can do to get him to pay his share without ruining my credit? I’m still paying this loan, and it’s accrued almost 5k of interested now.