My kitty won't stop yowling!

I have a 9 month old, neutered kitty named Nicky.

He will not stop yowling.
No matter how much attention I give him, how often I feed him, he will not stop.
Recently, he escaped out of my apartment into a shared storage area and my neighbour let him into her apartment to play with her two kitties. Now, he sits in front of the door and yowls.

I try to distract him with his toys, but he just wanders around the apartment, yowling away.

I was considering getting another cat, so he'd have a playmate, but I'm seriously going crazy with him.
It's all day long, all night long.

HELP ME.

My guess is he found those playmates outside and this cat could be spraying your door and enticing him. Does he have distractions in your home? A window perch where he can watch birds at a feeder outside? A cat condo, or cat ramps?

Have you considered getting a second cat to keep him company? Though that might backfire as well.

When he yowls what is the first thing you do?

When he continues yowling what do you do next?

Is he part Siamese?

Both the neighbour kitties are un-spayed females. I imagine he catches their scent and this inspires the aggressive behaviour that he displays practically 24/7.
I do have a cat condo, however he despises it. Occasionally, during play, he'll tear-ass up the tall pole, and hang there by his claws for a bit, but he won't sit inside the little cozy kitty house I made for him or anything.

All my windows are kitty accessible, but I live on the third floor of an old house, and there isn't much to look at, other than treetops.

When he yowls, I've tried giving him attention, petting him, throwing his toy across the floor to play fetch with him, picking him up and moving him, calling his name and trying to distract him... But it usually doesn't stop his meyowling.
When it continues, I try to ignore him, because I'm at a loss as to what to do, and am usually pretty frustrated by this time.
I used to put him in his kitty-carrier, but it got to the point that if he hear me shout "no!" he would go lay in it himself.

He could be part Siamese. I got him from a random home with a "free kittens" sign on their driveway.

Ahh those loveable "free kittens" Here is what I would do-

Buy some feliway spray and several comfort zone room diffusers by Farnam Pet. Spray the underside of the door and up the door in the frame about 2 feet high both sides. Plug the diffusers in.

Talk to the neighbor about the advantages of getting her females spayed. It is better health wise for both of them, unless your neighbor is an ethical breeder.

When Nicky yowls, pay NO attention to him. None, put on music, buy earplugs, whatever it takes, but don't pay attention to him. If you do, he will only train you even better than he has already. The minute he is silent, the very second, give him a tasty treat like Kitty kaviar or wild-side salmon Plain Brown Tabby Toys and Treats has good treats for kitties. Pet him when he is quiet, feed him when he is quiet, play with him when he is quiet, but do not respond to him while he yowls.

I'm looking into the Feliway Spray and Room Diffusers right now!
My mum gave me cat repellent (that smells disgusting and makes *ME* want to vomit, so I can only imagine how *HE* feels!) and I used it to spray under the door, and any place that was a "trouble spot" for Nicky, but it apparently didn't work.

My neighbour isn't an ethical breeder, I think she just doesn't want to spend the money to get the ladies spayed. I've had a female kitty, and they're so much nicer spayed!

I will try the reward method that you've suggested, haha, perhaps I can un-train myself.

Thank you!
I'll post my results in a couple weeks.