Praise be to Allah.
They can stop him from going for a voluntary Hajj, and they will not be sinners if they do that. But they do not have the right to stop him from going for the obligatory Hajj, and they will be sinners of they stop him. If he goes for (obligatory) Hajj without their permission, his Hajj will be valid regardless – even though he would be a sinner if he went for a voluntary Hajj without their permission. He also has the right to travel to seek knowledge without their permission.