The problem is he's twelve, verging on moving to being autonomous and he's kicking back ( which I think is quite normal) against you making decisions for him. I would personally give him more responsibility, personal responsibility that is. If he wants to be in a West End musical he's going to have to be good, so it would benefit him to practice but if he chooses not to that will result in him being unlikely to be cast, but give him the choice to choose to practice or not, then when he's not cast he will learn the glorious lesson of cause and effect, which will serve him very well. When I was a child there were no rules in our house whatsoever except be a decent human being, that sounds chaotic but it wasn't people learned and quickly, that if they did or did not do certain things there were always consequences brought about by themselves. Bring him on board the decision making processes of things that relate to him, otherwise this will get into a cycle of him being deceitful, you catching him, giving him a hard time, him feeling dis-empowered and then doing it over and over again in ever increasing degrees. Treat him more like the young adult he is growing into.
Porn is a difficult thing, it's your internet connection, you have every right not to allow him to watch that obviously, but he's going to somewhere because people at that age and interested and curious, so as Spath says explain Porn is fine and normal in degrees in adult relationships and you understand his interest but that it can set unrealistic standards and that's not what anyone's adult relationships are actually like, can be coersive, and you don't want him watching that in your house because of that.
I think if you continue to take things away, punish and assert your authority he's going to do it all the more, it's clearly not working, so I think you need to establish a truthful dialogue with him and set some steady ground on which to build an honest relationship. x