Hmmm... my first reaction is let him quit, and you get max VP to zero, but if that's not allowed... (I do think someone should always have the option of forfeiting, however).
Was he frustrated with you, your crew, his crew, or what? I'd settle him down a little, ask him what is so wrong, and go from there vs just calling him out on it (not that I think that's wrong, you'll probably get better results this way, tho).
Anecdote that may help: I suck at Warhammer fantasy, and one of our local players (who is an incredible player) and I were playing a game, and he was beating the pants off me. I am the worlds most competitive person, and was being a good sport about it (his words; not mine), so he told me somewhere around the time it became clear I was going to lose to just pick a goal- destroy a unit, have a unit alive at the end of the game, whatever- and try for that. If he is losing so bad, or doesn't know his crew well, you could suggest the same. I can't speak for everyone else, but I've played a few games where pretty quickly I was playing for a scheme and knew I was gonna lose. Just trying to achieve that makes the game enjoyable, and helps you learn.