I would go for another cat if you have not done so. Tim makes a good point that the introduction may NOT be 100% positive. However, in time, the cats will get along. It took Poopsie a LONG time to accept the fourth cat, Willie. But now, while Poopsie like to occasionally swat at Willie (she does that with all the cats), they get along very well. Willie and Poopsie will lay right against each other in our window seat. Yes, they may roughhouse, but that is just their instinct. On the other hand, your current kitty may be like my Munchie in that he was around other cats for most of his life. Munchie came from a shelter so he was exposed to other cats. As a result, Munchie easily accepted every cat after him that was brought into the house. He never was aggressive towards them at all. In fact, when we got Tooties, Munchie found her hiding in the basement. Tooties then came right out to Munchie and they bonded right away. As a result, Tooties made herself right at home in an hour or two when it takes most cats a few days to come out of hiding.
Lastly I feel that any animal is happier with another one of its own kind. We all know of separation anxiety in pets. Well, having another cat in the house of one cat is to their mutual benefit and the first cat will be happier in the end to have a playmate.
Somewhat off topic. I was at the Metro Toronto Zoo in the summer of 2000. I was absolutely FURIOUS when I walked out of that zoo. Why? Because SO MANY exhibits only had one animal!
My goodness, I could literally feel the sadness coming from the lonely animals. I about lost it when I was at the giraffe exhibit! That poor giraffe was so lonely. Good thing I did not run into a zoo employee. While I may have gone off, I know they have no control. It is the idiot director that allowed the animals to be solitary.
So with that in mind, I honestly feel that in the long run, after all is worked out between which cat rules the roost, they will be happier in the end with another cat around.