This sounds very hard op. You are definitely not alone.
And don't worry, I think lots of teens go through quite an unlikeable selfish phase. And everything is especially difficult for them, and us,v at the moment. They usually come out the other side ok! There's no rule that says you have to like your son all of the time as long as he knows you love him deep down.
During low moments, it's very tempting to give up but I don't think we can. Despite the fact they never seem to listen or take any notice of what we are saying, I think they need us there to keep fighting for them, even if the "enemy" is themselves.
As far as study is concerned, are you in touch with the school? Can you form a team with them to try and keep your son on track? Tbh, though, motivation really has to come from within him, so you could try talking to him about what he wants to do with his life, maybe do some research in to unis/colleges/apprenticeships/careers? Some teens become more focused on their studies if they know they need certain grades to take a certain path.
Or if you have already tried that, take him on a drive, tell him you appreciate things are really hard right now but that he won't feel good about himself if he just gives up. Maybe tell him you aren't going to nag him any more, it's his life and try and get him to realise he is in charge of his future?
Finally, I'm not a prude but I just wouldn't tolerate any sort of violent porn. Your are his parent in charge of the house. I would be sitting down with him and having a good talk about why it is frankly wrong in every way and demeaning to women etc etc. I would be changing the wi-fi password (once I worked out how) and making him earn back his privileges (maybe with study assignments?) despite his age. Sorry but I wouldn't compromise on this.
Good luck . It's not a nice situation to be in, especially right now as you can't really reach out for moral support from friends. If you need a break from it all though, try and take a step back and focus on yourself for a bit, or else you can get drawn in to a very negative spiral that won't help you or your son. 