I know someone suggested just leaving it alone but I think the OP mentioned that he has to do 30 minutes per night for homework. I assume there is some sort of log. I have taught my kids "creative reading log accounting" that if there is 30 minutes per night and 5 nights required that there should be 2.5 hours of reading per week and I am not going to worry if 30 minutes on Monday and one hour on Tuesday and he didn't do any on Wednesday. I don't consider it unethical to just put 30 minutes per night. Life doesn't always give you thirty minutes every night.
But if he wasn't reading at all, I wouldn't sign (my kids had to have it signed and the book listed) and would be looking for some reading materials that might grab his interest or even different ways to enjoy reading (audio books, reading aloud to him, shared reading, magazines instead of books, etc.). And I wouldn't worry about audio books because he has the skills to read and comprehend...he just hasn't found the joy.
