He knows who to call if he wants to sue.

A billboard worker wound up in the hospital Tuesday morning when he tumbled off a rooftop sign in Brooklyn that happened to be touting the famed personal injury law firm Cellino & Barnes, family and officials said.

David Fricker, 49, was working on the billboard around 10:28 a.m. near the corner of Church and Flatbush Avenues when he plunged 10 feet to a rooftop below, his step-son and FDNY officials said.

Fricker is now recovering in Kings County Hospital with a fractured back and a concussion, according to his step son and wife — who are now demanding answers from his employer, Lamar Advertising, on how their loved one fell.

“Rest assured I will find out because I want to know what happened and they’re not answering our questions,” Fricker’s stepson James Sanabria, 36, told The Post Tuesday.

Sanabria and his mother got the news from a secretary they know at the company, but no one there has called to officially explain what happened or to express any sympathy, he claims.

“We’re completely frustrated, my mother is completely upset and beside herself right now,” Sanabria said.

“He’s been with the company a long time, at least 14 years. I’m a little shocked and upset that they didn’t want to check on their own worker.”

Sanabria said he believes his step-dad was wearing his harness when he fell, because it was brought with him to the hospital and doctors had to remove it.

When he inspected the harness, he saw no damage to it.

“We know he had his harness on but no ones giving us a direct answer,” said Sanabria.

The family is waiting for doctors to do a CTE scan on Fricker’s brain to see if the bleeding has gone done.

He said it’s too early to know if his family will employ the services of Cellino & Barnes or any other attorney over the incident.

Lamar didn’t immediately return a request for comment.