Hi Lorraine and welcome to Pet Talk. I have 3 males and they get along fairly well together. Males usually do better in a multi-cat household since females seem to be more territorial. Since your male cat is still fairly young, I think that either a male or a female cat would do well. Like Christiansmommy said it all depends on your current cats behavior. If he's very easy going and laid back then he'll probably get along with either males or females. I would also recommend that the new cat be younger than him so he doesn't feel as threatened by it.

From my own experience I've found that males are more lovey dovey and easy going but there are always exceptions. I've also always introduced a younger cat to my household so that the other cats don't feel as threatened and so the alpha cat can retain his status. I'm sure you'll receive many other great responses because there are many knowledgeable people here. Good luck with whatever sex you decide. Please keep us updated.