imo, slip ups/relapses are not a requirement for recovery but they are very common. i also think maybe its time for you to focus more on you. nothing you can do or say will make him want to stop or his mom to stop enabling, he has to want stop for himself.
i lost my little brother to addiction so i do understand that you don't want to lose your husband to this but there really is nothing you can do other than began to focus more on you. maybe it is time that you take a step back and allow him to suffer the consequences of his own actions and this goes for the mom too. you can't change people but you can change how you allow others effect you and your life.
it took for my family to take a step back and allow me to fall before i became desperate enough to seek help for myself. you and your family are in my prayers.