Sometimes if you discuss the technique before starting to train you can find a mutually acceptable ukemi where you will get the feel for at least some of the technique and your uke will not feel threatened. If it is an issue that he is really sensitive about this of course wouldn't work. Even so, this is really more of a solution for you than for him, since he still isn't pushing himself to do the proper ukemi for the technique. Still, better half a donut than none at all... I must admit that I've only used this approach with those who are physically unable to take a reasonable fall from the technique, but willing to work out alternatives.