Agree with above although I think I get it that you are the one being whacked by your horse when he throws his head around. That is really dangerous as you could end up with a broken nose or worse.
Have a vet check teeth, ears etc to be sure he doesnt have a problem because generally speaking they do it because of pain or because of poor training.
When does he do it? Is it when he wants to speed up and you are telling him no?
It could be he hates the bit you are using?
There are many reasons for head tossing so it is important to understand the why before you try to correct it, and to do it in short order so you nip it in the bud so to speak.
It's not my horse. I'm borrowing my neighbors mare for awhile because I sold my mare and my gelding was having a complete melt down. So, his tiny little mare is over keeping him company and it's done wonders. She throws her head all the time. I've not ridden her as I don't think she's broke but it's just something she does. When you're near her, when you're just watching her from the window. She's a head thrower. They had a vet out recently so I believe she's been checked out medically. I think it's a behavior problem as the people who own her now got her and 2 other horses with the acerage they bought. They know nothing about horses but they're trying. I just wanted to work on this while I have her.
Now it is becoming a bit more clear. If she throws her head all the time she definitely needs to be checked. If there are flys or bugs, buy her a face, ear shield. If its not fly season then either its ears, or teeth bothering her.
If she throws her head as much as you say something is not right in any case.