Don't know if this will help, but it sounds like with the latest incident (arching back and hitting his head) that he's simply trying out new body movements. He hasn't been able to control himself like that before, and now he wants to test it out and see what will happen. He may try it a few more times before he makes the connection that it doesn't feel good!
At that age, they're getting very mobile - it's hard to prevent every fall. Try to relax. As he gets older and figures out that you freak out whenever he touches the ground in a less-than-pleasant way, he'll start to work to get that type of reaction from you. And he won't take those spills and falls well himself.
Ask your mom if she can remember you falling the first few times. I'm sure you did - and you survived. All babies fall. All toddlers fall. Most kids have broken a bone or two and survived just fine.
What's happening is normal - maybe not ideal, but normal.