None of the reports have given any precise detail on the exact nature of the injury. The consequences vary so much depending on what areas of the skull and brain have been struck and damaged. To complicate matters parts of the brain can be damaged by it's being shaken around in the skull so it's unlikely we will hear anything with any certainty for quite some time. Some types of injury, such as weakening of blood vessels, can lie undetected for days, weeks, even years.
Obviously we should hope for the best but it is a concern that his condition is apparently not stable. My hopes and best wishes go out to him, his family and friends.
Edit: BBC now report that an official statement from the hospital in Grenoble that he was in a coma on admission and that he has undergone some preliminary surgery. He is still in a coma and in critical condition.