My cat is definitely NOT the father of these kittens; he hasn't been outside (except for in my arms when I carry him outside for fresh air) in well over a year so that's an impossibility
I appreciate all of you taking the time to write about neutering, but my partner and I have chosen not to neuter our cat at this time. Whether or not we choose to breed him, adopt other cats, keep him as the only cat in our home, neuter him in the future, etc. is our personal decision as pet owners and when the time comes to make that decision, if I need further information I will start another thread. All I am looking right now is how to stop his howling in the middle of the night.
Thank you to the above poster for providing links to places that may be able to help this stray.
And incidentally, the longer the pregnant stray has been hanging out on my balcony the less my cat seems to be agitated by her. As of last night he's down to meowing/howling on and off for about one hour, the rest of the night he's back to sleeping on the couch by himself or in bed with me. Whenever he would get too loud (loud enough to wake me up) I either gave him a loud "Shh!" or clapped my hands. He seems to have gotten the message that this behavior is not acceptable and he's almost completely back to his "old" self.