Some points to remember:
1. First explain how the doctor knows you and for how long. Also, that he gives you his highest/strongest recommendation.
2. Discuss your performance when you shadowed this doctor and how it relates to your ability to be a capable and compassionate doctor.
3. Show that the doctor knows you as a person. Discuss how your personality demonstrates ideal qualities of a physician. Your ability to interact with others and patients.
4. Discuss any other details about you that the doctor feels was significant in giving you his highest recommendation.
Since you're writing this LOR, I think you should give yourself as many pats on the back as possible. It's a bragging fest.
Hope this helps. I had to write one of mine too. What a pain!
Keep it simple, professional, and don't be afraid to flatter yourself.🙂