If he is as addicted as you say, I wouldn't let him have 2 hours every school day. I would let him have 2 hours once in the week and maybe 3 hours each weekend day.
If it wasn't this, it would be something else that he would tell you that you are ruining his life with.
I'm sorry that you have an addicted DS, but I think once you get him out of the habit of playing every single day, it will be better for him, and for you. I don't think the addiction is your fault at all, btw. Some are like this. I have two DSs, elder one is not addicted, will not play for weeks/months and then will play a whole day and I don't mind that. DS2 used to be a bit addicted, but luckily has grown out of that. He's 13, like your DS. Doesn't really play in the week, but if he gets a new game, as he has last weekend, then he can play a bit more in the week, but now otw he only plays sometimes at the weekends as he's usually out doing stuff.
Does your DS do any activities? Football/cadets/scouts? If not, I would really get him involved in something so he wouldn't have enough time to play so much XBox.