"off his case" is a perfectly correct expression in written English.
It is often used to mean "finished working on a problem or task" or "finished considering something," as in this example sentence: "The detective was finally able to close the case, so he was off his case."
Suggestion: get off his case.
While Testaverde was answering questions, his backup, Ray Lucas, chimed in, "We're 6-2, get off his case".
On May 8th, Dick Cheney announced that his friend was "the best secretary of defence the United States has ever had", and warned critics to "get off his case and let him do his job".
If you think that word "arrogance" is too strong, consider the statement Vice President Cheney issued through a spokesman over the weekend: that "Don Rumsfeld is the best secretary of defense the United States has ever had," and that "people ought to get off his case and let him do his job". Let's see.
Get his teeth fixed so that his wife, a dental assistant, would finally get off his case.
You'd think that any dude in a six-month slump would opt for new look (say, high and tight) if only to get the fans and sports radio guys off his case — especially in a sport as superstitious as hockey.
He had boxes of documents and photographs that I began to report off of; his case hatched open all these historical records.
And yet, enough of the old Dr. Graboys comes through that a reader might conclude he would also be foolish to sign off his own case completely.
Mr. Iannucci said yesterday that he remembered being told by his supervisors to hand off his other cases and focus on convicting Mr. Shobey of raping several young girls.
Everyone does it, a sharp tongue, a big pair of fists, fierce makeup, confident clothes, pre-emptive tears...or in Eminem's case, writing a bunch of songs about which female celebs he'd like to make out with and telling the world off for getting on his case all the time.
Her lawyer was helpful at first, Mr. Brenna said, but backed off last year when his case was about to settle.
