I agree with SR.
Even of course you'd try to have a deal with his mom and agree that no one cleans after him anymore. Perhaps he'd finally find that cleaning service times are over now and he has to do it himself or he turns totally icky and messy (I believe most of people have their limit after which they cannot stand messiness anymore...
now he just don't need to think about it because someone else does it for him).
For example, if he can't take trashes out, let him keep it. Let trashes fall down to a floor, set your own garbage can and be sure that he doesn't use it. I believe he'll take the trashes out some day.
Cannot guarantee it works and you need to have nerves enough to stand messiness if you try that trick. I'd still try that myself if I had a situation like you've now. Or I'd try to try, I'm somewhat tidy person and don't know how long I'd stay in conditions like a dump.