I'm definitely agreeing with others on increasing his exercise, I hope you are thoroughly prepared for the needs of a full-grown Jack, which can be as high as that of border collies and other larger athletic dogs. Think flyball, agility, jack trials, jogging, biking. I've met a couple Jacks happy with a couple daily walks and a yard, but in my experience they are more the exception to what is typical, and as you never had the opportunity to meet his parents or talk to the breeder you have little to go on as to where they stood as representations of their breeds.
For now, maybe check with friends and neighbours if you can hire a dog-walker until your injury has healed? Get in some visits with a friend who has a yard and a doggie playmate?
If you use PB in a kong, use a very very small amount. Peanut butter is very high in calories (=more energy) and can cause diarrhea, especially in stressed dogs. All you want is a tiny droplet in the hardest-to reach corners. Iams puppy biscuits are also a perfect fit to pop in the small-size kong.