I love what I call a verbal cat, one that talks to you but I don't know that I could handle one that yowls a lot. Maybe he would rather be outside and that is why he is being so vocal? Could also be that he is either calling his humans or is just mourning them.

Boo yowls now and then, I think she does it to make sure I am still here but once I answer her, she comes right away and is quiet.