Your son sounds like me when I was young. Exactly like me, I acted out in different ways but the pattern is the same. The high performing sister, the divorce, etc.
I struggled for a long time to find what it meant to a man without a male figure in my life worth looking up to and attempting to emulate. The truth was there was always somebody there worth emulating in my life I just couldn't see it because it wasn't "cool" and it was my mother.
She was and is the hardest working person I've ever known. She was the one who made the man I am today, and I'm proud of it. The truth is that he won't recognize or understand what you are doing for him until later on in his life. He may hate or resent you for some of the things you need to do. He'll say thank you later, but I'll say it for him now. With that out of the way...
The first step is probably to get him evaluated by a mental health professional to make sure that there isn't something else going on first. That isn't a mark of weakness or that you think he's crazy, just that sometimes it can be good to talk to somebody who isn't your parents, he may end up being more honest with them than with you. This is to ensure there is not a learning disability or something else that needs to be addressed.
The second step is to be ruthless with his grades. You can't be his friend on this issue. You have to take away things when he fails and reward him when he succeeds. Set clear penalties and clear rewards for progress before school starts and set clear identifiable and attainable goals for him involve him in the discussion and creation of those goals and what he thinks would be an appropriate reward and penalty.
The third step is to get him involved in some others said a group of some kind. For me it tabletop role playing games, it does not have to be traditional sports. It could be a card game, or something else that he is interested in and wants to play. Forcing Sports and Music on me worked out poorly. But roleplaying games were perfect for me, it allowed me to be creative and spontaneous and build life long friendships.
The fourth thing is to talk to him about the Swastikas. Does he really understand the popular meaning to them, even if it he may be alluding to the traditional use of them as a good luck sign? Does he really feel as if they are an expression of his, of how he feels? If he thinks Nazism is 'cool' or 'metal' preferably take him to a holocaust museum. In lieu of that watch some documentaries together and ask him what he thinks. Is that who and what he wants to be associated with?
The fifth thing is to realize that for him, Stoicism may be just another passing fad as he struggles to find something that he can identify with. Next month it may be something else, or he might find it to his liking. It's impossible to know with somebody this young and with such a lack of experience.