As with any behavior pattern change, you have your work cut out for you. Chances are, this won't be easy. I would begin by having talks during the daytime about bedtime. You may want to get some good children's books about bedtime fear. I would also get a good shade for his room to block out the shadows.
Shadows can be very scary.
You might also play some music in his room or play stories on tape. When my children were young, I would read their favorite storybooks into a cassette tape and then they could play the tape at bedtime. You might also develop a checklist for bedtime. This gives routine and structure to going to bed. Finally, be firm.