Wish I could help with what to do - but I have a 14 year old cat that does the same thing. Whenever she thinks she is alone she yowls until you call her. Her eyesight is going, too. There is such a thing as feline Alzheimer. He may be getting that. I just hope my family is as patient with me when I start randomly yelling!
If you find any help - post it. I would like to know, too.