We've been dealing with head banging like this for years. My son is 7 now and does it much less---rarely, but he still has episodes of it. How old is your son? Is he verbal?
My son had very little language when the banging began at 2. The more verbal he became the frustration lessoned and the head banging wasn't as frequent.
1. They actually said carry a small pillow at all times to slip under his head and protect him. We did as much as we could of this as often he head banged on any hard surface including concreate.
2. When he is calm teach him stratagies like deep breaths, asking for help, and some other physical thing he might like to do like jumping in place up and down (which is safer). So it was hard going, but when he was calm and happy we'd bring up just talking about something we knew he didn't like and calmly tell him (acting out) stratagies and have him try them. It took a long time for this to "work" and not have the banging, but we still work on this to replace the banging. It has some.
3. Keep yourself safe. My son nearly broke my nose and cheek bones with all the banging he did into me. Keep a soft barrier for yourself too. I sometimes would wrap him in a blanket too in self defense if we were at home.
The main thing is to keep yourself breathing and calm also. It is hard not to react as it is disturbing to see a child do that to themselves or others or you. It is constant teaching that brings them out of it. We are nearly out of the woods and can see the light at the end of the tunnel. I hope you will be able to soon.

I also thought of one more thing that has helped and that is writing down instructions for transitions on a small white board. Giving him lots of time to transition helps reduce head banging episodes, but even tonight with lots of warning at bed time the lashing out began. He was just too tired. Sometimes, you just have to weather through those storms as calm as you can and know it should be better once he gets some good sleep! Good luck! (Yes, at 5 is when he nearly broke my nose and his teacher's nose too with head banging).

Hi Sarah! My son is 5 and yes it was much worse as a toddler. He would bang his head in his crib often. Now it's when frustrated or sometimes even over excited. He is slowly but surely speaking more so I pray over time to head banging will go away. I am considering sending a small pillow with him to school like you mentioned, Ty!!!

My son will sometimes thrash his head from side-to-side. This tends to happen most when he is in his car seat and when he is either tired or overstimulated. I distract him, and/or tell him to stop. If we are not in the car, he will intentionally 'bump' into things or people with his head or arms. I've been told this is a sensory seeking activity. I will ask him 'Do you need head squishes?' or I will just start massaging his head or engage in a good tickling session!