Help!! I am a computer illiterate. Here's my delima.?

My 14 year old is doing very poorly in school and I have limited his computer time. In order to keep him off of the computer, I have changed the pword, so that he can get on only when I see that he has his homework done. Here's the problem. When I do give him access to the computer, he sometimes goes in and changes the pword so that I can't get on. To him, it is funny, but to me it is very frustrating. Is there any way that I can prevent him from changing the pword to an unknown?