This guy is already in the right position to be restrained.
Lock some heavy steel manacles around his wrists and chain them to the bar above his head. Spread his ankles wide and chain them to the wooden posts. Lock a heavy steel collar around his neck. Fix a chain to each side of the collar and stretch them to the posts on either side and padlock them in place.
Anything else? Gag? Hood?
What do you think?