Ok, first things first, you have a JRT!! They are first and foremost a HUNTING dog, bred to go into the earth, locate and corner quarry such as Fox, Raccoon, Groundhogs, Badger, Oppossums. All critters that have teeth and claws that will use them against the dog, therefore jrts have to have a certain amount of aggression to do the job that they were bred to do!
Having said that, your pup is probably just honing his prey drives and hunting/engaging skills, which is normal, but not safe for the cat on any level.
Yes you can socialize him to the cat and teach him to leave it alone BUT be careful about using corrections, that can offend lead them to really direct aggression onto the cat when your NOT present.
In short, don't EVER leave your Jrt alone unsupervised with the cat because he could easily just kill the cat one day and it wont be the dogs fault. It has been well documented over the years of the people who ended up with dead cats because their jrt killed it. Even Jrts that were raised with the cat and they were friends for years.
It always amazed me when I was an offical Russell Rescue Rep the number of people who had no clue WHAT a jrt is or what they have been bred for and continue to be bred for to this day. The number one reason for people getting rid of their jrt was aggression to people/other dogs and other small animals etc, raising the dog in a permissive home and the owners not knowing how to recognize it or how to deal with it.
I have owned multiple jrts over the years, this info is based on being a breeder, rescue rep, hunting with my dogs and showing them in everything from conformation to performance events.
At this time I have 5 Jrts in the house, all except the youngster has proven themselves as a hunting earthworking jrt and ALL will call off a cat BUT I would NEVER leave one of them alone with a cat for any reason.
As for training, positive reinforcements work best, teach a solid 'leave it' with HIGH rewards. It takes a LONG time to train it correctly and it is something that needs to be consistantly reinforced with high rewards.
You are more than welcome to PM me at anytime. And here are some sites on JRTs that gives the facts about them and some good advice.
www.terrier.com
www.terrierman.com
Good luck.