Dear Mr. & Mrs. XXX,
I am glad to hear that Kazu is fine and he is doing well with you.
I understand there are two more weeks before the school starts on Sep. 3. I hope he is preparing for his new school life. If there is anything he should learn before the school, will you let him know about it? I guess he should get some stationary, learn how to go to school and/or where to shop, etc.
Thank you very much for taking care of my son at your home. In case he does anything to trouble you too much, please let me know.
Looking forward to hearing from you again.