Lol, all this talk of warnings.

I would send someone like this a letter (e-mail) detailing everything expected of him and ask if he is prepared to continue working under my conditions. I would outline that it is unpaid work but having good work ethics is a quality he should try and aim towards. I would also say that his work so far is excellent, his quality and attention to detail is above expectations etc, but his time management skills are severly lacking. So much so, that projects are getting too far behind deadlines. Ask if he is prepared to step up to the mark and display the professionalism that you know he can achieve. If not give him an opportunity to gracefully resign.

In everything you write be professional and civil.