The father of two young British jihadis who died fighting in Syria made an emotional appeal on Tuesday to his sons’ peers not to travel to take part in the war after confirming the death of the second son.

Abubaker Deghayes also made a direct appeal to the prime minister and UK authorities to create an “exit strategy” that would make it easier for young British jihadis wanting to return home to do so, and said that the withdrawal of his own passport had prevented him from trying to bring his sons back.

He was speaking after confirming the death of Jaffar Deghayes, from Brighton, who was 16 when he left to fight alongside his two elder brothers, Abdullah and Amer. Abdullah, 18, was killed this year fighting in Latakia province in the north-east of the country.

The brothers’ father told the Guardian earlier in the day that Jaffar, who was one of the youngest British jihadis known to have been fighting in Syria, had been killed on 26 October in the vicinity of the Syrian city of Aleppo, fighting forces loyal to the regime of Bashar al-Assad. The death brings the number of British jihadis known to have been killed in the conflict to 25.

His father said that Jaffar has been shot with a bullet to the head and had lived for 15 minutes before dying, after uttering the Islamic testament of faith.

Deghayes said: “The grief I have for my two sons cannot be described. My words fail to describe it.”

He said he learned of his son’s death on Monday from Amer, who sent him a recorded message from Syria. In April, Amer was shot in the stomach and watched Abdullah die under fire from forces loyal to the president as they stormed a hill together.

Referring to his surviving son in Syria, Deghayes said: “I cried my eyes yesterday and I really hope and pray to God that he will return safe to his homeland wher he lived all his life.”

Deghayes went on to say that his message was still the same: “All young Muslims, however young and naive, who thinks to go to Syria out of kindness and to try to make a difference – do not go.”

“The Syrians do not need fighters. They need weapons, they need quality weapons, they need governments to help them. Do not make this mistake. Do not go there.”

He went on to say that he had a message for the prime minister and the home secretary, Theresa May: “You have tried to stop me from going there to bring my sons back by keeping my passport by six months.”

Deghayes said that he could go to Syria if he wanted to and had been in the past to carry out relief work, but accused the British authorities of “criminalising people” through a strategy that they were carrying out.

Of current policies towards young British Muslims, he added: “Do not push them to be radicalised by groups like Isis who are out for revenge and thirst for blood.”

He urged the authorities to work with young British Muslims and also have an “exit strategy” that would enable those in Syria to return.

“They should be treated like a youngster who made a mistake. Not just put in prisons. The stick of the law does not work all the time. That does not work Mr Cameron. Please, please revise your strategy. We do not want to lose more of our youngsters.”

Ibrahim Kamara, another young Briton who was a friend of the Deghayes brothers is believed to have died in a US air strike in late September, along with three other Britons of Bangladeshi origin from east London. All were fighting for Jabhat al-Nusra, al-Qaida’s Syrian affiliate.