He still tours but if you watch video of it a lot of the time he's just sitting there at the keyboard while his (large and really good) band makes the music. Since he's so old his voice is a shell of what it once was. In addition to writing and arranging and producing great music, when he was young he had a fantastic voice.
I've read that he's been diagnosed with schizoaffective disorder, which means he hears voice, but keeps it mostly under control with whatever medications they give him. But he went a long time undiagnosed and "treated" by that crazy Dr. Landy guy, which didn't do much for him. It was only after they got rid of Dr. Landy that he got real medical treatment.
He has said that drugs put him over the edge back in the 60s, not that he did that much of them but I think he had borderline mental issues in the first place and the drugs were the final straw. It's the classic genius/crazy guy dichotomy.