Is Charlie a male and is he neutered? How does he react to other cats? My best scenario is to get a kitten, preferable female if Charlie is a male, that is the magic age. About 3-4 months. They are old enough to be on their own and yet if Charlie doesn't like them, these slightly older kittens can defend themselves.
I've had a disaster trying to put two grown cats together. Eventually I took the newest one back. She had been an "only cat" in the previous household.
There is a series of U-Tubes for Friskies called " Dear Kitten". That is worth watching.