We adopted a cat a year ago named Paddy and he seems to be very happy here but he too yowls on the deck multiple times a day. He doesn't seem upset when he's doing it and I think he's calling for other cats. A few times our only outdoor cat has come to the screen door of the deck when he does this and otherwise they are never together because she doesn't like him. But I think he's calling her when he does it.
My guess is that Bobby is calling the other male as you said and might be looking for a friend. That would be so cool if you could slowly befriend this other cat and maybe adopt him too. Are you able to offer him some food? He might be less scared if he knows that he can get food from you.