Former US president Joe Biden ’s prostate cancer has spread to other parts of his body and is causing him pain, even as he continues to speak out on public issues, his son Hunter said in an interview.

In a wide-ranging interview with the BBC broadcast late on Friday, Hunter Biden grew emotional as he discussed his father’s condition, describing it as very sad to watch.

“The cancer has spread, metastasized into his bones and further,” he said. “It’s very painful and it’s very debilitating in many respects.”

Biden, 83, was the oldest man to serve as US president and his age was a source of concern for much of his four years in office. He and his White House advisers have faced intense criticism for allegedly concealing the extent of his problems, especially after he was forced to abandon his bid for re-election after a disastrous debate with his eventual successor, Donald Trump.

The former president revealed his cancer diagnosis in May of last year, less than four months after he left the White House.

Hunter Biden said the diagnosis had been tough on the family.

“It’s really sad to watch,” Biden said. “The only thing that I’d say about my dad, about his health right now, is I wish he would complain more, because it’s not good.”

But Biden said his father remains “out there”, speaking publicly about the issues that are important to him.

“He’s still doing his thing,” he said. “He so believes in this country.”

Biden will publish a memoir, Promise Me, America, after the midterm elections.