A beloved retired FDNY fire marshal, in a race against time after being diagnosed with the coronavirus, learned Saturday night that he was not a candidate for an experimental drug he’d hoped could save his life, his family tells The Post.

John Knox, 84, of Rockaway, Queens, is in isolation at Mount Sinai South Nassau hospital in Oceanside, according to his son, Zachary.

Knox started getting sick last month and checked into South Nassau Feb. 26 after feeling dizzy and experiencing bouts of coughing and vomiting.

Doctors didn’t test Knox for the coronavirus until March 9 because had not traveled to any high-risk areas or been in contact with anyone who had, his son said.

The bad news came back Friday. Knox has been heavily sedated and on the ventilator for 10 days due to breathing issues, his son said.

“My father was a strapping young man who every kid in the neighborhood looked up to. He was a role model for everyone,” Zachary Knox told the Post. “Our biggest concern is that there is enough time to find something to save him.”

After reading a story in The Post, the Knox family had hoped to get their loved one an unapproved drug called Remdesivir, made by Gilead Sciences, which has been used in experiments to combat the Ebola virus.

But a family friend told The Post Saturday night that due to his acute kidney failure, he is not considered a candidate for treatment.

Knox was a member with the FDNY from 1960 to 1998, serving as a fire marshal for 34 of those years. Prior to that, he was in the NYPD for two years and served in Korea with the Marines.

Knox had been going to the gym up until a few weeks ago.

“That’s why it’s so disconcerting to see him lying in a hospital bed,” his heartbroken son said. “It’s extremely upsetting for all of us who love him to watch him laying there helpless.”

Knox is positioned face down in a special bed in order to breathe easier.

Son Zachary said his dad was so “proud” to be an FDNY investigator, that he always wore his fire marshal ring and the family had to “wrestle the ring off his finger” when he was admitted to the hospital.

Knox, who remained in isolation Saturday, also has a wife, three daughters and six grandkids.

The family, who had had been visiting up until Friday, have now been tested for the coronavirus and are self-quarantined while awaiting the results of the test.