It would be too obvious for me to write the entire letter for you, but I'll include a list of points that you could include in your own writing style:
* Thanking Lee for his letter, and saying that you are happy that he is having a good time. * Telling him roughly what you're studying at school (without being boring). * Write about what you would like to do when you leave school. * Finally, say that you miss him a lot and look forward to hearing from him soon.
