If this is your first job, and you're only 6 months in, you should definitely be making below the mean.
He said he makes below the average for his position, not necessarily his field.
He probably shouldn't have taken that little money, but there is nothing wrong with asking for a raise now. If they say no, he knows where he stands.
My advice OP, would be to start looking now. If you ask and they reject, you'll already have the ball rolling and your employer *should* be expecting you to leave anyway. Best case scenario, you get an offer within the next month and you can take that to your boss to negotiate.