If you mean specifically the forearm, then yes, it is quite unrealistic. Still, I wouldn't say it's impossible given the right conditions, like enough adrenaline.
i still say it's impossible. Your weakest point will break first, and that would be the wrist. Now if his wrist was lopped off and he kept on punching with the stump.. maybe.
But even then I'd imagine his elbow wouldn't allow it.