From your question, it sounds as if you are using this novel with just one student. If that is the case, you might actually like to think about allowing your student, with your help and guidance of course, to design his own writing prompts. If he or she has enjoyed the novel, which I am sure is the case, you might like to discuss what your student would like to write about.
Based on my own reading of this excellent and gripping novel, some starting suggestions would be as follows:
At what stage did you discover the identity of Wormtail?
What would it have been like for Wormtail to remain disguised as Scabbers for so long?
How does Harry feel and change as a result of having an adult who loves him and wants to look after him?
Why do you think that Harry chooses to hide what is happening from Albus Dumbledore?
In addition to such questions, there are lots of opportunities to take the issues in this novel and encourage your student to reflect upon them and relate them to his own life. Issues such as deciding when to lie and when to tell the truth, the importance of parental love, being in a situation where you are forced to disguise yourself and your emotions, the feelings of anger against injustice would be an excellent basis for a reflective journal based on the reading of this novel. Good luck!