Are we forgetting that he's got kipnapped by Penguin, he had no use being in that type of life. He went from carjacking to being a crime syndicate and just a few days...Anyone could crack under those circumstances. He does gain his confidence as the episodes go by, but the stutter doesn't get better. I think thats a good portrayal of stuttering( confident can only do so much, and it isn't a magic cure).