Thanks for your question. My name is XXXXX XXXXX I'd like to help you out.
I'm sorry to hear about your son's anger problems, he obviously comes from a very loving family. It sounds like he is a great boy in a lot of ways, and considering his many successes in other areas, I would anticipate that he will eventually be able to overcome these problems. It is not uncommon for exceptional young athletes to start to feel entitled, lash out, and to struggle with things that they don't enjoy, such as school work. In some ways he is probably extremely well adjusted and intelligent, but it also sounds like he lacks some impulse control and self discipline.
It's good to hear that removing the TV and stopping his football training for a week worked. Despite the fact that he reverted, that approach generally does eventually start to work if you are able to be firm, consistant, and stick with it. I know you mentioned that he has all of the IT gadgets that he wants, which is fine, however these are also things that you can remove if he continues to be defiant. Once you start removing things as consequences on a regular basis, it is possible that he may get more defiant before he improves. However he will start to learn the overall lesson that he does not get to have the rewards in life if he does not do his work and treat people respectfully. A lot of time threats of removing him from school or taking certain things away won't work until you actually do it and he sees that you are serious. While you may not have to resort to actions so drastic, you may have to take something away from him that matters to him in that moment as opposed to saying what you will take away. From what you described, it sounds like you are on the right track, and I would encourage you to stay strong and stick to that method.
If you are at your wits end or have been down this road many times before, I would certainly recommend having him meet with a child psychologist. These types of problems can occur for many reasons, and a child psychologist should be able to help you get to the bottom of this, as well as give you a better understanding how to help Mitchell after having met with him. From what you described, I would not assume that Mitchell has any serious problems, and there is a very high liklihood that with the right support and approach he will get through this phase.
I definitely wish you and your son the best with all of this. If there's anything else I can do to help just let me know.