I know this can be a problem. My son uses his computer too much, but so does the rest of the household. I restrict his often by simply removing the computer when I don't want him to use it. After all while it's his, we are the parents. And I control access when necessary through the router. With software on the router I can control when he has access to the internet, and block web-sites.
He has had his own computer since 6th grade (11). His 6th grade teacher told his class that they were encouraged to bring either an ipad (ipads had just come out & other tablets not yet out) or a netbook for their own personal use to school. He was experimenting with having the kids really use technology. While he couldn't require this I think the entire class complied. It was a bit odd because the kids were happily using technology a lot but then weren't allowed to bring these devices to junior high. I ended up buying my son a small netbook because I wasn't happy with ipads & as the types of games my son likes to play often only work on PC's.
My son has had his own computer since them. After 2 years I replaced the netbook since it really wasn't a very good computer and kept breaking down. My older DD didn't officially have her own computer till she went away to college, but effectively had primary use of an old one of my husbands by high school.
My suggestion would be a small laptop. IMO tablets are good entertainment devices and not as good for actual work.
Good Luck..
