ezo
Get off my lawn!
To be clear: is he "trying" because he wants this bonus, or just because he's trying to act in character?He is trying, poorly. He wants to get the bonus or plus, of course......but that comes with the risk of the negative.
Again, I would be fine with him doing the dull, robotic game play.
I don't....but if he tries for the bonus, he must also accept the bad side.
I mean, I would let him "do his thing" for now, tell him you appreciate his attempts at role-playing, but until he improves it is best to ignore the bonus/penalty for it. Just let him try. If he improves (hopefully) and is more experienced at role-playing, and you both feel comfortable with him trying for your bonus/penalty reward, then start doing it.
Until then, he can "role-play", but don't have it affect his rolls either way.
I don't know if that will work for you or not, but that's the best advice I have.