A friend of the British hostage being threatened with murder by Islamic State militants has made an impassioned videotaped plea to the group's leader in an attempt to save his life.

The appeal, in both Arabic and English and addressed directly to Abu Bakr al-Baghdadi, who styles himself as caliph of the self-proclaimed state, was posted on YouTube Wednesday, four days after militants threatened to kill the aid convoy volunteer from north-west England.

The friend, who calls himself Abu Abdullah, is understood to have witnessed Alan Henning's abduction near Aleppo last December. After describing how the taxi driver from Greater Manchester – "a family man, a good man" – washed cars to raise money to buy medical equipment for Syrians, he breaks down, begging al-Baghdadi to spare his life.

"He felt safe, relaxed and happy, believing as we did that no harm could come to him, or us, while he was in the place under your care – especially because he was bringing much needed aid to the brothers and sisters in Syria." He begs al-Baghdadi to "honour our amana" – a pledge of safety that Henning had received from his Muslim friends – and set him free.

Henning, 47, who is married with two teenage children, volunteered to join his Muslim friends on the convoy last Christmas. It is thought to have been at least his second trip to Syria.

He and his friends were surrounded by armed men at a warehouse a short time after crossing the Turkish border on Boxing Day. He was separated from the group, according to accounts that some gave to journalists after they were released and returned to Britain, and taken away.

They and Henning's family remained silent about his plight, on the advice of the UK Foreign Office, until he appeared in the group's latest video last weekend.

Entitled A Message to the Allies of America, the film first shows David Haines, the Scottish aid worker, being forced to deliver a speech in which he blames his death upon David Cameron and Britain's relationship with the United States. It then depicts Haines's murder by beheading.

The killer is then shown holding Henning by the collar of the orange jumpsuit in which he is dressed. Indicating towards his captive with his knife, the killer says: "If you, Cameron, persist in fighting the Islamic State, then you, like your master Obama, will have the blood of your people on your hands."

As well as making the video appeal, Henning's friends released a clip in which Henning explains why he has decided to spend Christmas travelling to Syria.

He says that joining a Syrian aid convoy was "all worthwhile when you see what is needed actually get to where it needs to go. That makes it all worthwhile.

"No sacrifice we do is nothing compared to what they're going through every day on a daily basis."

Cage, an organisation founded by Moazzam Begg to campaign for the release of prisoners in Guantánamo Bay after his own release from the US military facility, described Henning's kidnapping as a contravention of Islamic law. Begg was himself charged with alleged terror offences relating to Syria earlier this year.

In a statement, Cage research director Asim Qureshi said: "Alan Henning is not involved in any hostility to Islam or Muslims. Therefore, he cannot be considered a prisoner of war under Islamic law and should be released immediately. We believe there are no grounds for holding Henning prisoner or executing him."

In his appeal, Abu Abdullah goes on: "Alan was moved by the suffering of the Syrian people, in particular the children. He devoted all his free time, raising money and awareness about the suffering. He washed cars to raise money, he collected aid, he talked to everybody he met about the crimes committed against the Muslims in Syria.

"In December last year he missed Christmas with his own family, to spend it helping our brothers and sisters in Syria. We all remember one night, on the journey to Syria, some brothers were so tired they wanted to get a hotel. But Alan said: 'No, not me, I'm going to sleep in the van, and the money I save, it can go towards the Syrian people.' This is the kind of man he was.

"He is a family man. He is a good man. Like you and like us, his only interest was helping to relieve the suffering of brothers and sisters in Syria. Please do not take his life to pay for the crimes that the international governments have committed. Alan ignored advice from the British government and took the risk to travel to Syria to help the people.

"We read every day about innocent brothers and sisters being killed in the air strikes, but this man is innocent of all these crimes. He has no affiliation with any political agenda.

"He has shown through his actions, and his love and concern for our brothers and sisters that he is not an enemy of Islam or the Islamic State. We plead with you to show him compassion and mercy as he showed compassion and mercy for the Muslims of Syria."

Finally, he says: "We beg you to tread the path of justice and show him the compassion that Allah, praise be upon him, placed in the hearts of the believers and in your heart.

"Please show the world how merciful Muslims can be. We know you're not scared of the creation, only the Creator, so please, please, please release Alan."