Cats stop grooming for the following reasons:
1) Extreme discomfort or pain (can be the kidneys hurt....or can be arthritis in some cats for ex.)
2) When they are overweight they often can't REACH well enough to groom properly
3). As cats age, they lose teeth. Teeth are how they
pull apart the tangles. Without all their teeth, they cannot get rid of the mats-they simply don't have that capability.
It's important that you maintain his grooming for him.
Mats can cause severe open wounds underneath the mat that can become infected, painful, and dangerous to his overall health.
Poor baby. He sounds like he's in quite a bit of pain.
Cancer is horrible and kidney disease quite painful.
I'm glad he has you and that you are devoted to helping his last years be loving ones.