He did not say a word to the police at all. They never got close enough to engage him before the paramedics swept him away. Any observed difficulty walking could easily be explained by his cervical fracture and a pneumothorax. As I personally was not present, I cannot say for sure if he was or was not under the influence. It just seems that the police immediately leap to conclusions, regardless of whether they have proper evidence for them.
If there is no other obvious reason for a car to spin out of control and have a serious wreck with physical injury, then OUI is often suspected. The officers may have received reports for erratic driving or may have received information from whomever called for the emergency personnel. The paramedics' observations are admissible as to intoxication if the court orders the records to be produced. If the blood tests and observations come back without indications of alcohol levels that would constitute OUI, then he will be all set and the charges will likely be dropped.
I'd like to add a response to the "operating to endanger" which is that falling asleep at the wheel is presumptively negligent, and the statute under which "operating to endanger" is codified includes negligent driving. If he had, for instance, begun a new medication that had an unknown side effect, then he was involuntarily intoxicated, and that would be a defense. Getting into a car when one is over-tired is a choice, and if they get into an accident, it's common to charge them with operating to endanger. There doesn't have to be an intent to be a danger, just an intent to get into the car and drive in that state.