Your son was more then lucky, or more then just the helmet helped him to survive that. I'm glad he did for both of your sakes. I've know people who died in motorcycle accidents due to brain injury at 1/4th of the speed your son was going and they too were wearing helmets!
Having said that, I don't think helmets should be required by law for anyone over 21...BUT...I also don't think safety belts should be required for anyone over 21 either. I'm an adult and should be able to choose what safety precautions I want to take, I always wear a seat belt or a helmet because I want to, but if you don't fine.
I hate laws like this because they enforce seat belts but not helmets, either require seat belts AND helmets or don't require either. Why is one class of motor vehicles forced to comply with a safety law while another class is not? Just no common sense.