You and your mother are going to need a lot of patience with these problems. He has a dementing illness that is incurable and is effecting his brain. His short
term memory is severely effected. He is unable to remember what you are telling him about his bathroom habits. It appears that keeping the trash can in the bathroom and emptying it after he has used the bathroom is the best idea for his situation.
As for the grunting and groaning, has he been evaluated for pain? Might he be uncomfortable? Call his physician to see if giving him a regular dose of some over-the-counter pain medication would be safe. If it is, give him the medication on a regular basis for a week to see if their is any change in this habit.
If he can still eat and is not in any danger of chocking, something soft and sweet to suck on may be helpful. If he enjoys music, earphones playing his favorite music might be helpful.
Let us know if any of these suggestions work as I'm sure others are facing similar concerns.
Best of luck!