Form what you say it is as black & white as this:
His visa was issued under the terms that he had left Britain before, or by, the 19th Feb. If his employer was allowing him to remain in work then he should have applied at least a month before expiry with the backing of the employer.
Although from what you have told us, I bet the IND are as confused as we are!
Surely if he's your boyfriend, he needs to come back on a fiance visa, unless you are not thinking of marriage, in which case he'll probably struggle to get admitted again.
