I am not an employer. But I have been an employee like most people. I don't have some leadership experience though. One thing a good leader cannot do is sacrifice the greater good for trying to be nice. You DO NOT have to be nice with this person. Just professional. You may need to earn this person that their job is in jeopardy if that is the case.

You can only retrain someone so much. You can find out if there is a problem, or find out if this person would be happy elsewhere. Are they just a bad worker? Poor work ethic? Is his job one of those ones where it is mind numbingly boring jobs that makes it hard to focus? Is he the only one who does this? It is hard to say. The big thing to find out is WHY he is missing. But he firm. Don't be afraid. If the guy is a sub par worker he doesn't need protection. He needs to be consistent with the quality of his work.