The parents of Peter Kassig, an American aid worker held hostage by “Islamic State” (IS or ISIS) terrorists, have appealed for his release in a statement and video message, the Guardian reported Saturday.

On Friday, a video was released of the murder of a British aid worker, Alan Henning, the fourth western hostage killed in recent weeks. The video warned that Kassig would be killed next.

Ed and Paula Kassig, of Indianapolis, Indiana, said through a spokesperson that their 26-year-old son was taken captive on his way to the eastern Syrian city of Deir al-Zor on October 1, 2013.

He was doing humanitarian work through Special Emergency Response and Assistance, an organization he founded in 2012 to treat refugees from Syria, the family has said, according to the Guardian.

Kassig, who Pentagon records show spent a year in the army as a ranger and was deployed to Iraq from April to July 2007, converted to Islam while in captivity and has adopted the name Abdul Rahman, the family representative said.

In their appeal for his release, his parents mentioned the Muslim holy festival of Eid al-Adha, observed on Saturday.

“As Muslims around the world, including our son Abdul-Rahman Kassig, celebrate Eid al-Adha, the faith and sacrifice of Ibrahim, and the mercy of Allah, we appeal to those holding our son to show the same mercy and set him free,” the Kassig parents said in a written statement quoted by the Guardian.

In the video, Paula Kassig addressed her son in a personal message that she said she hoped he would see.

“We are so very proud of you and the work you have done to bring humanitarian aid to the Syrian people,” she said.

Kassig served during the Iraq war before being medically discharged, the family has said. After leaving the army, he became an emergency medical technician; he travelled to Lebanon in May 2012, volunteering in hospitals and treating Palestinian refugees and those fleeing Syria’s nearly four-year civil war.

Ed Kassig, who works as a school teacher, said in his part of the video message that his son could not control U.S. policy.

“There is so much that is beyond our control,” Ed Kassig said. “We asked our government to change its actions, but like our son, we have no more control over the U.S. government than you have over the breaking of dawn.”

The beheading of Henning, condemned by the British prime minister, David Cameron, and President Barack Obama, was the fourth such killing of a westerner by Islamic State, following the deaths of two U.S. journalists,  James Foley and Steven Sotloff, and a British aid worker.