you know i agree with liesagain. as long as he have you to provide his basic needs, all he has to be concerned with is getting his drugs.
i know you don't want to tell your 80yro mom but before he goes to live with her, maybe you can at least let her know what is really going on and give her the chance to decide for herself whether or not she wants to let him move in and if she does, what can you do other than invite her to alanon. the choice is yours.
i'm glad that my family left me alone to my misery, i think i'd still be using, in jail or dead, if they hadn't. i'm a mom and i know having to make this kind of decision has to be one of the worse ones to make, but what if you knew that him suffering the consequences of his actions would save his life.