My DH was really nervous about the idea of UC with my 4th pregnancy ... it wasn't until the last week or so that he actually picked up New Nativity II and started reading birth stories ... and then it was only a few stories that were near nightstand. He tends to really freak out when anyone in his family gets hurt or sick, so it was a constant worry for me. I prayed about that one thing more than anything else during my pregnancy ... I just didn't want him causing anyone tension (mostly ME) during the labor and birth.
One thing that helped, which I had to keep reassuring him about, was that he didn't have to DO anything in terms of 'delivering baby' ... in fact, I gave him some specific jobs that made him feel better ... like he had a plan. "Keep the baby warm - get those towels and keep them coming" so it was a focus for him. I knew he wouldn't have to 'deliver' the baby because babies get born without anyone handling and assisting - especially in the water. When the time came, DH asked, "What do I do?" and I answered, "Nothing" and our baby was born into the water and my hands. I don't think DH truly got it that he didn't need to play doctor or perform or DO anything.
For me, it helped being prepared to know that I would do anything and everything that needed to be done ... I was mentally prepared for a solobirth even though my heart was set on a family birth.
If your DH likes to read or is willing to read birth stories, send him links so he can read lots of them! it may help to print out a bunch of them from Laura's website or ChristianUC.com or Birthlove.com ... let him read a variety of stories so he sees there can be a variation of normal in length of labor, pushing stage, birth positions, etc. It will also help to get an idea of what other Daddies were doing during the births of their babies. If you really see him in the role of baby catcher, then print out those stories where Daddy was totally involved ... if you see him with a less active role, find stories like that.
Congrats on your upcoming UC.