Has your cat had his T4 levels tested lately? Yowling at night can be one of the symptoms of hyperthyroidism. So can weight loss despite a hefty appetite. I would get that tested as soon as you can.
Many senior cats also develop varying degrees of dementia as they age, just like people do. Perhaps leaving a night light on for him would help, or calling out to him when he's crying.
What are you feeding him by the way?
"To close your eyes will not ease another's pain." ~ Chinese Proverb
“We must not refuse to see with our eyes what they must endure with their bodies.” ~ Gretchen Wyler