Hangon, that's a plot summary?
You have a setting, but no conflict, we still have no idea what the story's about. Is it an intermafia war? An intramafia war? Is it about him trying to work his way up through the ranks? Is it about him trying to get out? Put a little thought into it.
Also, why the hell would an American mafia have a Japanese guy as the leader, in fact, why are the characters Japanese, anyway? It's not about the Yakuza, so they're probably Japanese on one of your whims, which means they shouldn't be.
Somehow, I don't think the mafia would openly recruit kids as young as 15. 17 I can swallow, 18 probably more likely, but fifteen? Not gonna happen.
Vegas I can fathom, so you're ok on that count.
In short: if you wanna make it work, stop projecting yourself onto your fantasy of a mafia manga, do some research about the mafia, and actually come up with a plot.
How'd I do, Marty?