The Bangladesh origin UK family of Abdul Mannan was forced and tricked into going Syria against his will, Mannan's son has claimed.
Shalim Hussain, son of the 75-year-old man, made the claim in an exclusive interview with ITV News.
He said his father was forced into a van and taken to Syria against his will.
"My Dad is confused, sad, crying every day. He is not in peace," Shalim said.
He claimed the family was in Istanbul when a group of men came to their hotel room to check their passports before taking them downstairs one by one.
When just the grandfather and his wife were left, they were told to leave their children and grandchildren behind and to return to Britain, Shalim said.
But when they refused to leave, Shalim claimed the men bundled the pair into two waiting cars.
Muhammed Abdul Mannan, 75, his wife and 10 other members of their family, from Luton, have not been seen since leaving for a trip to Bangladesh in May.
A picture apparently showing Mr Mannan in Syria emerged on social media last week, and a statement released by IS claimed the family were "safer than ever" now they had joined the terror group.
