The thing that helped me get over my fears was being forced on to Mantis. I was afraid of both heights and inversions, but my friends forced me on it swearing that I'd have a good time. I did. Mantis was the newest rollercoaster at Cedar Point at the time and (at least I thought) the biggest, so after I got off the ride I knew I could face Raptor, Corkscrew (I was VERY scared of inversions), and Magnum (heights).
Anyway, I'd say that if you are really good friends (so he'll do what you want him to), have him ride a small ride first w/o inversions (Blue Streak), then a bigger ride w/o inversions (Gemini), and finally a big looping coaster (Mantis or Raptor). That's what got me hooked.
*** This post was edited by cpfan on 6/8/2001. ***