to the mission for ceasar then ask ceasar to let him go. i didn't do that so idk if it works.

Doesn't work. He has to die, according to Caesar and if you don't make a decision he gets crucified when you leave. I did manage to let him escape with a stealth boy and flee the scene. I ran for the exit of the tent, then for the exit of the camp, then out of Cottonwood Cove to safety without using one stimpak or med-x. I think I was wearing Lightweight Metal Armor and was around level 13. Really, the only hard thing is making the exit of the tent without the Praetorian Guards punching you to hell.