Mr Anderson was identified in court on Monday as being a British citizen from Banbury, Oxfordshire.
The closed-door session heard he is “suspected of committing a set of particularly serious crimes that pose a particular public danger to the public”.
The court added that the decision could be appealed.
Earlier this week, Scott Anderson, Mr Anderson’s father, said he had begged his son not to go to Ukraine.
“He wanted to go out there because he thought he was doing what was right,” Mr Anderson told the Daily Mail.
“I’m hoping he’ll be used as a bargaining chip, but my son told me they torture their prisoners and I’m so frightened he’ll be tortured.”
Mr Anderson said he was sent the video by his son’s commander and was left “in complete shock and tears”.
“I could see straight away it was him. He looks frightened, scared and worried,” he added.
