A friend of mine is actually conducting sleep research and says that 8+ hours is oversleeping and is not good. The optimal is 7-8. The only way way 9-10 is ok is if that what you naturally sleep. But on average, 7-8 is optimal and 9-10 is oversleeping which is unhealthy. If you listen to your body, it'll tell you what is good. So if he is forcing the 9-10, that is probably too much. It is also very important to get the same amount every day. Getting 6 one day, 10 another, and then 5, followed by 8 is not good. It's far better to get 6 hours every consistently than to shake it up.
Hope this helps.