He sounds like he would be a prime candidate for Prozac. If you can find a vet behavioralist in your area they will be able to help you. One of my guys was VERY aggressive to another cat (so much so that I was in at the vet on occasion). I tried all kinds of behavior modification, the amitriptyline (Elavil), and I was finally desparate enough to go with the Prozac. Once it kicked in, it made a world of difference in his behavior. There are a couple of other furry ones who have had these problems. If you search for Prozac you will be able to find their threads.

Good luck, I know how hard this is on you.