I'd second the notion that Jack will probably require some counseling to get his mind back in order. His time on the island will, ultimately, be as traumatic for him as it was for any of the boys, perhaps more so, and he will have to do some emotional work to return to "normal". This might take months, might take years...
After the savage experiences on the island in which Jack engaged, it would seem difficult, if not impossible for him to surpress his darker nature. So, unless he undergoes psychotherapy, he will act in inappropriate ways.
Talk about a speculative question! I have always wondered about what would happen to the boys after their rescue. Would they proceed to Australia (their initial destination) or would they be returned to their parents in a more dangerous England? I always hoped that some of the boys--particularly Jack and Roger--would have to answer to their crimes of murder, attempted murder, and the torture of the other boys. However, rather than being sentenced to some form of juvenile incarceration, they were probably resettled in Australia to wait out the nuclear war that was already underway. Jack's crimes were probably never revealed, and he may have returned to music, possibly as a choral teacher where he could once again exercise his authoritarian rule over a group of inexperienced youths. Jack's blood-lust for hunting would be hard to shake, and I pity any woman who would fall for such a controlling animal; but Jack may have been able to overcome his past and adapt to a normal social life, and as a juvenile, he probably deserved another chance to do so.