The trial of the parents of a Muslim convert who travelled to Syria heard he had obsessive compulsive disorder which led him to lose focus at school.
Jack Letts left home in Oxford at 18 for Iraq, then moved to Syria, raising fears he had joined Islamic State.
John Letts, 58, and his wife Sally Lane, 56, are accused of sending him money despite warnings from police they could face prosecution.
The pair deny three charges of funding terrorism.
The trial at the Old Bailey heard Jack Letts had been "very personable, engaging and humorous" young man who had a good relationship with his parents but had become more reserved in phone calls after travelling to Syria.
A police statement by John Letts said: "He had a phase being obsessed with football and would sleep next to his football. The same thing happened with religion.
"He started to become interested in what it is to be a Muslim some three years ago.
"Jack would want to outdo people. If he was to attempt something he would have to try to be the best at it."
He described his son as a pacifist but said he became upset by the suffering of people in Syria.
Mr Letts told police his son went to Jordan to learn Arabic but, before he left, his friend Anwar Belhimer expressed concern that Jack had been associating with someone with extreme views and that he might travel to Syria.
In March 2015, police visited Mr and Mrs Letts's home and warned them they risked prosecution if they sent their son money or property.
Mr Letts told police he had no reason to believe his son had been involved in fighting, having said he was not with Islamic State.
The trial continues.