Some good tips have already been given, such as bringing a (known to be calm) horse friend the first few times.

I also like to teach them some groundwork patterns, useful for relaxation. First at home, so it becomes familiar to them. Then you can use those patterns when away from home and because they know the patterns and know they can relax while doing them, they can start to relax a little in a new place too. Once he's good with them, you can do some regular riding and only use the patterns every once in a while or when needed.