Doctors in Hong Kong operated on Jason Leung Song-xue last night after the teenager - the sole surviving child of the Leung family held hostage in Manila - was flown back to the city, still in a coma. The surgery started in Tuen Mun Hospital at about 10pm to relieve the pressure building up due to damaged and swelling tissue in Leung's brain, amid a public outpouring of wishes for his recovery. The operation, which involved temporarily removing the front of his skull, was led by Dr Dawson Fong To-sang, the hospital's neurosurgery chief. It was still under way early this morning. The acting secretary for food and health, Professor Gabriel Leung, said it was estimated to last less than five hours. The 18-year-old suffers head injuries, most likely caused by a heavy object. He and his mother, Amy Leung Ng Yau-woon, who lost her husband and two children, arrived on a medical flight yesterday afternoon, the final hostages from the bus to return. Gabriel Leung said the teen's reading on the coma scale just before being taken onto the plane had improved from seven to eight. The scale, which measures the responses of unconscious patients, runs from three to 15. Leung said his condition was stable before the surgery, registering normal vital signs and cardiovascular functions. The flight from Manila had had no effect on his condition. It would take four to seven days after the operation for the pressure in the brain to go back to normal, he said, adding that no other apparent bodily injuries were found. Hospital Authority chairman Anthony Wu Ting-yuk also said before the operation that the teen's heart and lungs were functioning properly. Former schoolmates of the student are praying for his recovery. Ernest Jeffrey Wong, 18, a graduate of the Hong Kong Management Association David Li Kwok Po College, where Jason studied for five years, said former students were preparing a service for him. Wong said the 'kind, polite and caring' Leung had shared his burden when he was in need. 'A member of my family was very sick and I had no time to work on a group project. Jason took everything into his own hands and never complained.' On a Facebook page, 'In remembrance of the Manila hijack victims', created by the government, Chief Executive Donald Tsang Yam-kuen called on residents to send their wishes or prayers for Leung. Another Facebook page, 'Leung Song-xue, you have to live', was supported by more than 4,000 users. Lawmaker Dr Priscilla Leung Mei-fun, a friend of the family, asked on the mother's behalf whether it was possible for the government to arrange mainland experts to help. Secretary for Security Ambrose Lee Siu-kwong replied: 'If you have anyone in mind, I am very happy to discuss it with the health minister to see what can be done and how to arrange,' though he said mainland doctors needed special arrangements to practise in Hong Kong. Meanwhile, two other victims are stable after receiving treatment at Prince of Wales Hospital in Sha Tin. Yik Siu-ling, 34, whose thumbs and lower jaw were shattered by a bullet, had a four-hour operation to remove dead tissue. Yik would need reconstructive plastic surgery, the hospital's deputy chief executive, Professor Philip Li Kam-tao, said. Joe Chan Kwok-chu, 45, had surgery on his hands on Wednesday night. Vessels in his right hand, more seriously hurt, were reconnected and circulation had improved, Dr Bobby Ng Kin-wah of the orthopaedics and traumatology department said.