How do I file with a working college student

My 19yo son lives with me, a single mom. He attends community college full time and works part time. His employer will end up paying my son just over $12K by the end of the year and they currently do not take out any federal taxes. Last year I still filed as head of household because he didn't earn much money and was still in college full time. He will continue to be a dependent of mine until after he graduates college?

1. Will he be required to file separately from me or will I still be able to claim him since he's still a full time student? 

2. Should he have his employer start taking out federal taxes now or wait until he is no longer in school? 

3. What happens if he takes a break from school for a term, or falls below full-time status?

Any guidance regarding how I should file and what steps he should take would be greatly appreciated.