From my experience and from what I have read, nitrous oxide (laughing gas) is better/safer than the benzos many sedation dentists use whether it is by pill form or IV.

There are a few studies about using Nitrous Oxide during TBI events that have been published. They don't appear to indicate a risk but somebody with a better understanding of research data may make more sense of the result. I googled Nitrous oxide and TBI.

Since he is a few years past his injury, I would not be worried.

I'm interested to know what med they used to keep your son in a coma if you know ?