What do we do?

My husband & I are young, so our credit isn't the best. With that being said, my husband's former boss helped us by co-signing for us to move into this apartment. Since then my husband has been paying the bills & rent ON TIME. The guy now wants off the lease but due to my husband not making the amount required, he can't. He is now asking for us to put in our 30 day notice.. mind you we have a child. We really can't leave our child without a roof over their head, obviously, so we don't have any intentions of signing him off. It has now gotten to the point where this is an everyday conversation, & it's very stressful. What do we do? I feel so lost. Obviously everything was explained to the guy when we signed, he knew there was a possibility of this happening. *** We don't have anybody else to sign. We've talked to the landlord, she said she's not a mediator(obviously, I understand) & to figure it out ourselves.. This guy is just so persistent. She told him herself, we've told him, he can't get off. It's just down to everyday phone calls & texts, which honestly is stressful.