Start with some groundwork.
I don't suggest lunging because it can actually get the horse more worked up then calmed down. Appylover gave me this link and this is a good website about the cons of lunging a horse Round Penning
What I suggest doing is the approach and retreat tactic. Go and play with the sturrip....pretend to mount, mount than dismount, etc. Whichever you choose, the second he shows any nervousness or starts to tense up, stop and turn away and walk away (or run away). With you retreating each time he gets nervous he realizes that it's not going to hurt him. And slowly he'll let you go further and further.
Don't force him to allow you to mount him when he's fearful....it's bad for him and you can get hurt if he takes off again. Take it slow.
Horses have two sides of their brain...a left side and a right side.
The left side is their thinking side and play drive...and their dominant drive
The right side is their natural instinct drive, their fearful side.
Sounds to me that your horse could be more right brained than left brained...if that is true, then definitely do the approacch and retreat