Could you persuade him that this isn't a confrontation - it's a chat. Is the problem that he doesn't know his boss very well? Could he perhaps do a combo of a chat and a letter so go up to his boss and say that he'd like to have a chat with him at some point about his wages and he's made a few notes for him to have a think about and get back to him?

Reassure him that we ALL find this difficult but it's not going to turn into an argy bargy. It's really hard to find good people these days; your boss will want to hang on to him - much cheaper than finding a replacement even with a pay rise.