The man, who serves on an Alberta military base, says his son in Winnipeg goes by the alias Harun Abdurahman and runs a Twitter account that sides with the Islamic State of Iraq and Syria (ISIS).
The 56-year-old man agreed to speak to CBC News on condition of anonymity, fearing for his safety. He says his son went through a difficult childhood, and converted to Islam in 2008 while living in Ontario.
"Harun just started getting radical last year," he said. He found out when CSIS agents paid him a visit in December.
At that time, CSIS considered Harun a "radical extremist."
Agents pulled out a file three centimetres thick, documenting Harun's tweets and retweets.
"Some things made me want to throw up," the father said. "People beheaded, he's commenting on them like it's some big joke and he's applauding their actions. There was picture of Christian kids being assassinated and he said they deserved it."
It came as a complete shock to the father, who identifies himself as white and Christian.
"It was just horrifying to know that this was what my son doing," he said.
"How can my son be into something like this? He grew up in a Christian home, we took him to church," he added.
On Monday, CSIS agents called him again to tell him concern about his son is escalating because of the volume and seriousness of his tweets. They said he was being monitored 24 hours a day, seven days a week.
"Well here you've got your national security force, if you will, monitoring your child. How would you react to something like that? I didn't know what to say," he said.
Bill C-51 would allow suspects to be detained based on less evidence, and lets CSIS actively interfere with travel plans and finances.
The young man who answered Harun's phone didn't want to be interviewed yesterday, but earlier in the day Harun tweeted: "I may be in some very big trouble."
His father, who has a 15-year military career, only has sporadic contact with his son through text messaging. At one point, he lost contact for more than two years.
From average kid to ISIS sympathizer
The father first saw changes in his son when he was only seven years old. That's the year his mom got sick and died of brain cancer.
"It was like he turned out the lights and put a 'Do Not Disturb' sign on the door," he said.
He was withdrawn, unwilling to talk about his grief or accept the psychological help his father said he offered.
The depressed boy turned into a defiant teen.
His father recounted a meeting when his son was in Grade 7. Harun told a police officer, a truancy officer and his principal they could not force him to stay in school or stop him from smoking dope.
"'All you can do is arrest me if you find it on me, and good luck with that,'" the father recalled him saying.
The turmoil grew until Harun turned 16 and left their Ontario home.
"He went and lived with social services at some halfway house and they finished raising him," the father said.
Harun returned to live with his father as a grown man four years ago. Harun told his dad he had cleaned up his act and converted to Islam in 2008.
Harun didn't appear to be outwardly religious when he moved in with his dad, who had been transferred to Winnipeg a couple years earlier. But he did fast for Ramadan and ate halal meat, something his father went out of his way to purchase to make him feel at home.
"We were loving, accommodating, respectful," although he admits hoping the conversion was just a phase.
"When he was living at home, he was very secretive; a lone wolf. He didn't bring friends over, never talked about where going and what doing" he said.
Today, communication between father and son remains sparse and strained.
The father fears the boy he loves may be lost forever: "He says he wants to move to live in an Islamic state, he doesn't want to be a Canadian."
"He's gone, he's lost, I can't help him," he said, adding he hopes the Islamic Association of Manitoba can get through to him.
"I want other parents out there to realize that we all don't know what our children are doing behind our backs. We want to think we know. We want to think we have control but when the bottom line comes right down to it we really don't."
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