Ben Carson can stay in the race as long as he wants. There is nothing that says he has to get out.
But the hard truth is, Ben Carson has ZERO path to the Republican nomination. 2 polls OUT TODAY show him at the very bottom in New Hampshire. A UMASS poll released today has Ben Carson in 8th place in NH at 3%, and an ARG poll has him in 8th place in NH at 2% (33 points behind Trump)
What he should do after the New Hampshire Primary Tuesday is drop out of the race rather than keep pretending that (with a few changes) he's headed to the nomination.