Calm not mounted, terrified when mounted?

i have received a new horse in my stable, he is ok whn i work with him in the arena he is great but as soon as i put a saddle on you can see the flames coming out of his eys because he is extremely terrified. when i rode him the other day he tried to run off with me but i jumped off and he stopped immediatly. he is not afraid of the surroundings like cones poles etc.... i just dont know what to try... i changed his bit, the saddle, i took him out for a trail ride...same reaction. so i stopped putting a saddle and just started from scratch as he was a colt. any suggestions??