You can't help him. The bandages you use to try to fix him come off. If he wants help, he will get it.
I would not spy on him simply because I have learned (and it took time) that spending all my life trying to "help" my daughter did nothing for her and hurt me and the others in my family. We can not help our loved ones unless they are ready to be helped. And we can not talk them into it. It has to come from them or all we do is waste our money.
The way your stepson is still yelling at everyone else and guilting you, in my opinion seems he is not ready to take control and responsibility of his issues.
Those two weeks in a hotel probably will only delay the inevitable and Grandma will have spent more of the money that she needs. Lots of drug users also will be in that motel with him...common.place for addicts to end up.
Lastly, from Nar Anon I learned that our talking too much to our illogical kids only sets us up for abuse and does not change them. Like you said, your husband has said the same thing to his son one thousand times. He did not have to say it again. His son knows and will still manipulate everything his way. He just wants to remain in control.
I am very sorry that you are on this nasty ride. We have all been there. Most of us have decided to detach from the craziness because we have no control over it plus we just can't keep doing.it.
Good luck and hugs.
