Hi, this has got to be so frustrating when you have tried your best and nothing seems to be working. Add that into his problems at school, and, well....the xbox is just a symptom, not the cause.
I am also the CL on the add/adhd forum - https://www.medhelp.org/forums/ADD---ADHD/show/175
He shows many of the symptoms of either add or adhd. And this would also explain why your discipline measures have not worked and will not work. As Specialmom said.....you need professional help! A psychologist or psychiatrist, and hopefully one that specializes in add/adhd. I also tend to wonder if your husband is also on this spectrum....which might make getting help for your child a bit harder.
Here is a link to some general symptoms of add/adhd - https://www.helpguide.org/articles/add-adhd/attention-deficit-disorder-adhd-in-children.htm
There are effective ways to discipline, this link will give you some ideas - https://www.additudemag.com/behavior-punishment-parenting-child-with-adhd/?utm_source=eletter&utm_medium=email&utm_campaign=april&mc_cid=e53872b895&mc_eid=34d357d554
I hope this helps or at least opens your eyes to the possibility that this is more then just an Xbox infatuation. Please ask if you need more info. Best wishes.