If you can, why not tie him to the outside of the trailer? I would be leary of letting a horse loose in a two stall horse trailer, simply because most of them are not nearly wide enough for a horse to turn around in...he could wind up injuring himself if he tries.
That's just my humble opinion on it. I tie mine outside the trailer if I can't find a decently priced 'horse camp' for her. That way the horse can shift around quite bit, without the worry of slipping or over bending his body in the trailer. I eventually want to get one of those portable stalls for when I travel, that way the horse can move around freely and lay down without the fear of getting stuck there.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."