Our poor baby! He evidently decided he could do skateboarder tricks without a skateboard! He does things like that, he tries to do things he is not really physically ready to do because his little brain just doesn't get that yet :)
As parents and teachers of children we are always worried that our children will do this, try and do something they aren't ready to do in some way, either mentally, physically or socially and hurt themselves. It's extremely hard to be brave enough to let them try and stretch. And of course there are those times when we do have to say "no, that is too dangerous".
When your child has special needs it is easier to be scared and worried. After all, we are in a situation we may have no experience with and we may not know anyone else who has had these experiences either. And in the end we will drive ourselves crazy, should we have let him/her try? And if they do get hurt, we will berate ourselves forever more for allowing it.
This is one of the reasons I like to see listserves and other parent groups. There is nothing as helpful as other parents who have been in the same situation giving us support and advise when we need it. Although if your child has an unknown disability, knowing what the name of the disability is doesn't really change who your child is, but knowing other parents who share some of the same issues that you do can help.
As for our grandson, I think a padded room might help :)