Hi lucky girl, welcome to Pigeon Angels!
Buttercup was very lucky to have been found and adopted by you!
It was great to hear about his progress and how he's become a member of the family!
How old is he now, does he eat and drink all by himself? It would be great to see a photo.
As for getting him a companion, are you sure he's a boy? For a couple of years I thought I had a male pigeon, until *he* laid an egg!!!
The thing is, a couple or two hens can live together in harmony, but two males will fight all the time. If he's got lots of attention and the freedom of the house, he may just be happy to have a human family. Consider also that if you get him a companion there will be twice as much poop to pick up, and Buttercup is likely to lose some of the freedom he's been enjoying, because with a new adult pigeon in the house you won't be able to leave the window open or the new one will fly away.
Pigeons will only consider your home as theirs if they were raised there, or formed a special bond with you, or became the mate of a house pigeon.I would urge you not to buy a pigeon from China Town
... Even if on the surface of it you'd be saving one from ending up on the menu, you'd also be encouraging the trade by buying one from them. There are lots of beautiful, tame pigeons waiting to be adopted in some organisations near you, and if you want to get Buttercup a companion, that would be the best choice. Plus you might be able to take him with you, observe how he reacts to other pigeons and they to him, and that might give a clearer picture of what sex he is.
You could approach Mickacoo: http://www.mickacoo.org/
or WildCare in the SF/Bay Area, and I'm sure they'd be able to help and advise you.
Good luck, and keep us posted!