I’m sorry to hear about what you’re going through. I have no idea what the condition is that your son is suffering from, so forgive me if my advice appears somewhat ignorant.
I think it’s actually normal for a 15 year old to be rebellious in nature. Some are more so than others. Usually, if there is a calm atmosphere at home, that improves their mental state.
Kids at that age are finding their own feet, finding where they stand. And they’re bombarded by the music industry and gang culture- being told that somehow being “hard” is how one obtains a sense of power. I think being angry at him would just repel him even further tbh. At that age, the last thing a kid wants, is to be told what to do.
I think you as a father shouldn’t give up. I can understand it must be frustrating to have exhausted so many methods but I’m afraid you are the father so you simply cannot back out in his tarbiya.
Perhaps you could repel his attitude with kindness and fairness. If he insults you, tell him that you forgive him because you love him and he’s being ignorant. Be patient with him. Sometimes, the greatest form of da’wah is your actions. Even though you may not directly be dealing with your son, overtime, he’ll pick up on your behaviour (and your wife’s), your consistency in salah, your akhlaq with your family and neighbours and so on.
Seeing as he’s the oldest child, you could maybe give him a sense of responsibility. You could give him tasks to do for his younger siblings and reward him verbally (maybe even with an occasional gift) for his acts.
Above all, never underestimate the power of du’aa’. Guidance is from Allah. So continue asking for His help. Remember, this life is a test. Your wealth and children are a test.
Please also remember that the Prophet (صلى الله عليه و سلم) never raised his hand to hit even a slave. Your son isn’t a slave. He’s your son. You’ve done a big wrong by punching him like that. As much as I can sympathise with you with the display of his attitude and behaviour, I’m sure you’re old enough to know that what you did was simply wrong. And really, I wouldn’t be surprised if he now turned around and did the same to his kids in the future, cos afterall, pops did.