> This has caused a lot of disruptions in the company, and I'm receiving recommendations to encourage this millennial to quit

This line is completely off-topic and should not be there at all. It leads me to believe there is a personal bias at play that’s having an influence on your decision making.

This young guy is busting his ass to learn and more importantly, apply what he’s learning. And you have the audacity to disregard his effort based on a meaningless time-frame?

Younger workers, in general, are getting tired of the “serve your time” game that employers are playing. He knows his value to the company, is willing to take it elsewhere if you don’t respect his efforts.