OP you are in WAY too deep, you've wasted too much of your time and money giving the kid a chance which he could give a shit about. Send him. Don't waste another day with him. He either isn't old/mature enough to handle he job, or he isn't a machinist period, never will be. Way too many times shop owners thing everyone can be a machinist, and wants to be. Not the case. It took me years to finally see the light with that truth.
And the line......"It seems to take.about 2 or 3 to find a good one to keep" ......what a hoot! If that's the case you have NO idea how lucky you have been. Today most times it can take dozens of trys to find one keeper.
if you can even find any to show up for an interview. Many will call and "plan" to come for the interview, not many show up.