Did a mock interview for my final-year son. He'd like a different interviewer next time

Campus placements start at his college in a few weeks, and last weekend he asked me to do a practice interview with him. I've sat on the other side of that table for most of my career, so I agreed happily. Forty minutes later he was annoyed and I was worried. His answers were fine on paper. But he couldn't say what the companies on his list do, and he answered 'tell me about yourself' with his CGPA. I told him all of this, probably too directly. He says I was 'harder than any real interviewer will be' and that he'd rather practise with friends. He may be right that I'm not the best person for the job. How did you help your child prepare without becoming the problem?