A sudden change in behaviour clearly needs investigating and you seem to have found the cause. If you can afford to get it looked at by a vet that would be the best course of action although there are some great antibiotic powders on the market to help wounds heal.

It sounds like it is going to take a lot of time and patience to get Jack's confidence back in the big wide world again. Just a few minutes each day with lots of praise and treats so he associates going outside with good things not something nasty happening to him. Poor little fella - hope he feels better soon