He's old, Kinyonga, really really old. It no longer matters what he was fed or if he has a good light. He may be suffering, but he might not.
Melvin, I would suggest you just make him comfortable. Give him thick towels and low natural branches with bark that are fairly thin--around the size of his legs and a little bigger. You might want to keep the towel moist. Don't worry if he doesn't eat. He looks like he has quite good body condition for his age. My gut feeling is it is not kidney disease because other animals I've had with kidney disease have wasted atrophied muscles and look really, really sick. I would think a reptile would exhibit the same body conditon/symptoms. He doesn't look sick at all, in fact he looks great in your picture. Maybe he is just getting weak. They will grab their own legs if they can't grab a branch, which he might not be able to do if he is so weak. Maybe he fell and hurt himself and will get over it. If you think the time is imminent, don't worry if he stops drinking. Dehydration is a blessing during the process of dying.
Melvin, I think he looks pretty darn good for 6 years. He's got a big fat tail, big fat chunky legs and feet. He's got every one of his toe nails and he looks hydrated. He looks like you've taken excellent care of him throughout his life. Help make his life easier for him physically. You should be proud of the care you've given him. I'm really sorry your friend is not well.
I do not understand why anyone would suggest anything but palliative care for a 6-year old chameleon who is losing his strength. Very few people can afford what it might take to keep this animal going for a few extra weeks or months. This is not one of those chameleons that is broken to bits. He's old. Sometimes you shouldn't treat animals or people for that matter other than making what time they have left good time.