I have a similar situation (recent) to relate :)
Last summer I was asked to work with a four year old arab/morgan pony mare who had gone badly ring sour but was also never trail ridden because the previous rider was lacking confidence (to the maxx!) . She was afraid to bring her out of the ring because this mare was known to be spooky. Eventually it got to the point where she would not do anything for you in that one ring.
So I started with ground work first. I would lunge her before the ride then take her out to our larger arena (where she had never been). For the first few weeks, I could only do walk trot with her. If she bucked or refused to walk past the gate, she would work harder! She got the idea soon enough that she was going to be ridden no matter what, and she better just do what she was told!lol.
As a coolout I would walk her up the road (in hand) or on the trails. She got to the point where she would work well in the ring, and then LOVE the trailride/coolout afterwards because she recognized it as the end of our ride. So now she thinks of it as a BIG Treat when I just bring her out for a trail ride.
So for your guy, I think the suggestions that other people have said are good. They're all basically the same idea, just different techniques.
Good luck :)