John said the family believed their son was dealing with mental health issues in the months before the attack.
He had wanted to self-harm and had attempted to take his own life, he said.
“We were worried about what he might do to himself.”
John urged other parents not to be afraid to be “a bit intrusive” into their children’s lives, adding: “Try and have a proper chat. It’s difficult to find the time sometimes because you’re wrapped up in your own things.”
In a statement, a government spokesman said its thoughts were with the victim and all of those affected by the “terrible events” at Thorpe St Andrew School.
“We are committed to ensuring all children are safe from radicalisation and threats online,” they said.
“Prevent plays a crucial role in this, moving more than 6,000 people away from violent ideologies since 2015.
“In addition, social media platforms must be able to remove illegal content and stop children accessing harmful material by law – or face penalties of up to £18m for failing to comply.”
John acknowledged that his son was “looking at a long sentence” when he appears before the court again in December.
“We will support him as best we can, try and see that he gets the mental health support he needs and try and get back how he used to be,” he said.
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