Howler's mention of Pokemon got me thinking. You talk about him playing his computer games. Is it primarily just one game he is playing?

If he has a game addiction, many of the behaviors would fit with it. No or minimal issues at school would be because it is inaccessible (but he could be somewhat distracted there.) He may even say school is plenty challenging, because he wouldn't want additional work that might take away from it.

It would also be in line with defiance and attention issues.