I haven't contacted him for a while. What message should I send?

If you don't keep in touch, the relationship will definitely fade!

Since he is a doctor, of course, you should ask him some related questions so that he can get to know you from some things in his life, not to mention that you have admitted that he is a brother. You usually care about him as a little sister first, but you should do it step by step, but you should call him every day from the beginning, contact him once or twice a week at first, and decide the opportunity to chat or meet next time according to the topic of the conversation ... that depends on your own thoughts.