I think you can get another cat, but I wouldn't advise getting 2 new cats at once. He has had the house and you all to him self and is not going to be too happy about "intruders" taking over. I always have at least 2 cats, I think they are less destructive if they have a buddy to play with, and are just happier. He is going to let which ever cat you bring into the home know that he is the boss. I think the transition may be easier with a younger kitten, that is less threatening to him. You have to remember to give him lots of love and to let him dominate the baby within reason. It is hard to let the natural order of things take place but they all settle in much more quickly if you allow things to go along where he thinks he is in control. Of course you will save the little ball of adorable fur, and hug her and kiss her and fuss over her while he is happily eating a bowl of some yummy special cat food
. After 4 or 5 days they will be all cuddled up and he will be her new Daddy.