Have you tried calling him see if he can hear you or hear anything at all? Many
cats lose their hearing when they get
old, and start yowling because they can't hear themselves. Also, it could be a matter of orientation. He is old, so his senses may not be working very well. If an animal feels disoriented and lost, it will cry/howl until they feel safe. Seeing you makes him feel safe because you’re the owner.