What has usually happened (as you've perhaps seen on the old threads on this site) is that he needs to come clean to INZ (as in, list what there is, frankly) about what does come up in the police check. Either with the application in a cover letter, or when the CO asks for his comments, he needs to give the background to the incidents in question. This is often a matter of saying (in a lot more words and detail), 'I was younger then, and stupid not to stand up to what the crowd were doing, and of course I can see now this was a big mistake, as you can tell from the way my life has turned around since.' He needs to be humble and apologetic. He will need to say in what way he is now different. He will need to find people who know him now, who have some standing in the community, to give him a character reference.
And then you hope for the best.