I got the Internet for my 15-year-old son and set rules for use on school nights (to 10:30 p.m.). I often catch him chatting after midnight.
I have told him I'll have the Internet service turned off if he doesn't abide by these rules, but he continues to sneak and chat. We live in the country 40 miles from town and there is nothing else to do. He does well in school, but I'm afraid this will interfere with his schoolwork.
It is wonderful that your son is so interested in the Net, but his grades will start suffering if he continues to stay up late at night. At this point, you need to follow through on the consequences that you have set up. Either have the Internet service discontinued for a short period of time, or, in order not to punish yourself and the rest of the family, move the computer into your room or a room you can lock him out of. Tell him what you are going to do and what you expect of him, and then do it.