You are making two false assumptions. First, his previous numbers weren't even that bad for being the youngest player in the league but you talk about those numbers like it's proof positive evidence that he's a bust.
Second, you seem to assume that his month and a half stretch of showing improved numbers across the board is more than likely a hot streak when the plausible explanation is that he's simply developing.
To answer your question, at this point he seems like an extremely high risk prospect. He could continue to figure things out and become a prototypical power hitting 3B or major league pitchers could exploit his approach and he's a guy who flames out after some cups of coffee. But I think he has a pretty good chance to become an MLB starter (2 WAR player) and will put that as his 50th percentile projection or so?
But there is obviously a long ways to go as he's just 21.