I doubt that he will much. I've seen many people that react afraid when getting hit in the face train for years. They're getting better to some extent, but they never come to the level of tolerance like those people that didn't have much trouble with getting hit from start on. The reaction and ability to get used to it is very much depending on predisposition (be it genetics or early childhood imprint). And let's not forget, we're talking here about getting used to get punched by heavy weights who can put people to sleep with one punch.
I think that if he had started boxing when he was younger, he would be up to a very acceptable level now.... but he obviously didn't. I also don't see him getting picked on or beating up very often growing up. He's obviously a very tough guy, but if you're just not use to taking the punches, that's not something you're going to learn over night.