Perhaps if you wrote the letter for him and he only had to sign it it would be easier to get this done. Tell him if he refuses to sign it you will pursue it with legal action and you know he cannot afford that. That it would be in his best interests to sign off and be done with you rather than hiring a lawyer to defend himself from your legal action.

It's like getting divorced. Best to signoff early than pay all the money to the lawyers and have nothing left for yourselves.