Sueh Miah, 37, is terrified Abdul Rakib Amin, 26, could kill or be killed after the former Aberdeen schoolboy was seen in a recruitment video for the Islamic militants, urging other British Muslims to join the war.
Mr Miah said in an interview published yesterday: "Don't kill anybody, don't do any terrorist acts, don't do anything that's against Islam, come back home."
He added: "We will help you as much as we can, we're not angry with you. As long as you're safe, just come back."
Mr Amin is among 500 people from the UK fighting in the conflict in Syria and his appearance in a recruitment video, which emerged last week, came as a huge shock to his family.
He was born in Bangladesh and moved to Aberdeen when he was around ten years old, attending Sunnybank Primary School and St Machar Academy.
As a teenager he was a gifted boxer whose hero was Muhammad Ali and as he got older became more religious, returning to his home country to care for orphans.
Meanwhile, Alex Salmond has reached out to Muslims at the start of Ramadan.
The First Minister issued a statement yesterday wishing Muslims everywhere a "joyous and blessed Ramadan."
He wished followers "all the best" for the month of fasting that they traditionally take part in.
Last week, at First Minister's Questions, he said authorities would adopt a "zero-tolerance approach" to anyone who encourages hate crime against the community.