(NewsNation) — Each morning, the U.S. national anthem echoes from the walls of detained American citizen Paul Whelan’s Russian prison cell.

“He gets up every morning and sings the national anthem, I think as much to irritate the guards as to keep his own morale up,” Whalen’s brother, David Whelan, said. “And I think he is determined to survive, but I think you have to wonder how long he can continue to do that.”

Paul Whelan, a former U.S. Marine, has lost about 20% of his body weight since he was detained four years ago on suspicion of being a spy. His family says he was in Moscow attending a wedding.

Though they can speak to him on the phone, 2020 Associated Press photos of Paul Whelan inside a Russian courtroom are the most recent references his family has for what he might look like today.

On Wednesday, the family learned that another American, WNBA star Brittney Griner, would return home in a prisoner swap; the Russian government, however, is not budging on his case.

Griner had been serving a sentence in a Russian penal colony since February. Customs agents said they found cannabis oil in her luggage at Moscow’s airport at the time, leading to her incarceration.

Her release Thursday was part of a prisoner swap for convicted Russian arms dealer Viktor Bout, the so-called Merchant of Death.

Now, Griner’s wife Cherelle Griner said, the Griners will advocate from the U.S. for the release of other Americans held as prisoners in Russia.

“BG and I will remain committed to the work of getting every American home, including Paul, whose family is in our hearts today as we celebrate BG being home,” Cherelle Griner said. “We do understand that there are still people out here who are enduring what I endured the last nine months of missing tremendously their loved ones. So, thank you, everybody, for your support.”

Once Brittney Griner returns, David Whelan suspects she’ll have an outsize influence to draw attention to other U.S. citizens detained abroad.

At the end of the day, however, media influence and public sentiment aren’t likely to have the same impact in Paul Whelan’s case, his brother said.

“That’s unfortunate for Paul and for the wrongful detainees, but I think it’s great news if Miss Griner can bring her name and her energy to making sure the U.S. government is proactive and being more assertive to resolve these cases quickly, like they did in her case,” David Whelan said.

As the Whelan family continues to wait, it’s Paul’s kindness David Whelan said his family misses the most.

“I hope that when it comes out the other side, that these rituals … the singing and other things that he does to keep his mind squared away, the survival training to us from the Marine Corps will help him to still be the person that we knew before he got there,” David Whelan said.