I would agree with everyone here - we recently (about 3 months ago) got a young female to keep our (then) 18 month old boy Riley. However, we wanted a cat that would be a bit younger than him, so he could show them the ropes and be the boss, but not a very young kitten, which we felt neither we, nor Riley, would be able to cope with!
We ended up getting Skye, a 4 1/2 month old female, but we took her on a 3-week trial basis (with the agreement of the resuce place), just in case things didn't go well.
It took a while, but with patience and making a big fuss of our big lad, it all went very well, and just 3 months on they are practically inseprable. She's active enough to keep him on his toes, but relaxed and grown up enough not to drive him to distraction
Good luck whatever you decide - but take your time making your decision, and choosing the new cat. Also, there are great tips in the behaviour guide section about introducing cats (as I'm sure you know!).