I happened to have one of these mesh back supports in my car and on an instinct attached it to the head of the chair the child was banging his head on. He LOVED it! He was able to still move his head back and forth and meet resistance when he went back, without actually banging his head on something.
Since it's mesh and elastic, the material gives quite a bit, which protected his head as he banged it and made sure he never actually made contact with the chair itself. The nice thing is, they're also super inexpensive - you can get them off of Amazon for less than $5.http://www.amazon.com/Fellowes-Office-Suites-S…/…/B000MUSOZ6 (I'm not affiliated, just providing a link.)
Let me know what you guys think! As always, I'm not a professional, just writing from personal experience - it went well for this boy and I thought I'd share the idea with the rest of you!