My cat hates my roommates cat

So I got my cat Pippin (3M) when he was a kitten from the shelter. He was born there so he had his siblings and his mom for the first three months until he was adopted by me. He was neutered shortly before I adopted him. He’s always been really social, friendly with people and other cats. I had another cat (2F) that he adored and cuddled with all the time. He and a friends cat (2M) love each other and play and wrestle constantly when they are together, which is for a day or two every few weeks. I also have a kitten Maxwell (4moM) that he is very friendly with, they play a lot but he is gentle when they play. He has always been high energy with a destructive streak but until now has immediately gotten along with pretty much every cat that he has come into contact with, and even some dogs. Looking back, all cats that he has met were kittens when he met them but he has still remained friendly while they grew, so I know that is probably a big factor. 

Roommates cat Renji (5M) was a stray until he was about 7mo and got taken in by my roommate, and was neutered at around 8mo. He’s really sweet with RM and I, but can be kind of a grump sometimes. RM had another female cat for three years that he got as a kitten, and while it took about a month for them to get fully introduced but he grew to love her, although he had this strange thing were if they were separated for even like an hour to take her to the vet, when she came back he would hiss at her for a few minutes before he warmed up to her again. He generally likes Maxwell, the kitten, although he tends to get annoyed with his high energy.

THE PROBLEM

Pippin hates Renji, and the feeling is mutual. I have never seen Pippin be aggressive like this before. We’ve tried to gradually introduce them and it there has been no progress at all, if anything it has gotten worse. It has been since August. We’ve tried letting them meet under the door for weeks and they would play with each others paws under the door so we decided to let them meet face to face. Things went well for a few minutes, Renji pretty much ignored Pippin but Pippin kept following him around and trying to get him to play and batting at his tail (which Renji really does not like) until Renji swatted at him and they got into a fight. (Renji broke his leg years ago so he can’t jump very well, which means it makes it hard for him to get away when Pippin bothers him.) So we put them back in separate rooms and waited longer, tried to let them meet again, pretty much the same thing happened. We got a wire dog crate (like this: ) to put one of them for periods of time so they could be in the same room and get used to each other that way. When Pippin is in it, Renji ignores him and just walks around the house like normal, but when Renji is in it, Pippin will just sit and stare at him, and sometime tries to bat and swat at him. We also use the Feliway scent diffuser to try and make them calm whenever we put them in the same room. After about a month of this we let them out together again, but within about 30 minutes Pippin pounced on Renji and they got in another fight. Luckily I had put soft claws and a cone collar on Pippin to protect Renji. We’ve been doing the kennel thing since before early December, and there has been no progress it seems. Last week I accidentally didn’t shut the door all the way on the room Pippin was in and he got out and attacked Renji and they got into another bad fight. We are at a loss. I’ve given up on these cats ever being friends, now I just want to find a solution to make them not fight anymore. I know Pippin is the instigator and they would probably be fine if he would just stop antagonizing Renji. How do I get Pippin to just leave him alone? I would really appreciate any ideas y'all have. 

Boy, it looks like  you've tried just about everything!    I'm wondering if maybe a calming collar on Pippin might help.  The active ingredient is a pheromone and the theory is that because the collar is closer to the cats nose, and the level is constant, it is especially effective.  There are some complaints about these collars - some people say they are not as breakaway as they claim to be, plus they have a strong odor and many have a coating that gets everywhere.  Still, a lot of members have had success with them, so it might be worth considering.  Here's an assortment that show up on amazon:  http://www.amazon.com/s/ref=nb_sb_n...=calming+collars+for+cats&tag=&tag=thecatsite 

You could also try Composure treats or liquid to put into Pippin's food.  http://www.vetriscience.com/index.php?l=product_list&c=24  

Two other possibilities:

Bach Flower Remedies:    http://www.bachflower.com/rescue-remedy-pet/   

Spirit Essences:  http://www.spiritessences.com/category-s/1876.htm

And finally, here's an article on Cat introductions.  It does sound like you've been through a fairly complete introduction process but perhaps something in it will give you an idea of something else to try.  For example:  did you do any scent swapping?  That's an important step.
[article="32680"]How To Successfully Introduce Cats The Ultimate Guide  [/article]
 Good luck with this.