After a big adrenaline dump it can easily take a couple of hours to calm down properly - I would make him wait until at least 5.30 until you address this with him, because he will be more able to take in what you are saying then, and you will be better able to model the calm and self-discipline that he desperately needs to learn.
I suspect that your son does not fully comprehend the consequences of his actions (most young people don't, as violence is very unrealistically portrayed on TV/film/computer games etc).
Kicking you in the knee could have severely incapacitated you (this is why it's a popular technique in self-defence). Please put some ice on all the bits that hurt, and go to the walk-in/minor injuries unit/A&E/whatever you call it if it gets worse. Knee attacks can tear ligaments, muscles, tendons, cartilage and break bones. That's why they are not allowed in martial arts competition. I sincerely hope that you are at most bruised, but if you'd suffered what I just listed you could find yourself unable to drive, or walk with aids, for months and possibly years.
Punching you in the head? People's heads are very fragile. Had you fallen over backwards as you tried to get away from him, you could have hit the back of your head and suffered life-altering injuries, or death.
I definitely would explain the events to your son's sensei, as s/he needs to know in part so that they can keep a close eye on your son and make sure he doesn't do anything similar to other children in the class, and in part so that they can reinforce the "this is for respectful competition, self-defence and the defence of others, and NOTHING ELSE EVER" aspect of martial arts practice. Also, if your son knows that other people he cares about will find out what he's done, he may be less likely to repeat it.
When you speak to your son, I would explain about the danger of what he has done, very slowly and quietly. I would tell him that I would be letting his father/sensei know too - because you have a duty to be truthful to them. Whatever you decide is an appropriate punishment, let him know at the time and stick to it.
Frankly, I'd consider discussing it with his teachers as well, see if you can get some support at school.