I might be wrong, but if he had been legally determined to be mentally unstable it would have shown up during the instant background check.
I believe you are wrong, there is no method that I know of for reporting this in a manner that NICS can use.
I for one see the horrible failures in our mental health-care system on regular basis. As a defense attorney and prosecutor I've come to the conclusion that MUCH of the drug abuse in our rural county is related to self-medication. I've dealt many times with people suffering from major depression, bi-polar disorder, and schizophrenia. I believe that most of them abused street drugs and alcohol in order to deal with their symptoms...one of the more disturbing was the schizophrenic that told me he drank because it quieted the voices.
Generally I believe that there are no gun laws that can or should be enacted at this point to make us safer, with the caveat that any person committed voluntarily or involuntarily for mental health issues is not (apparently) reported on NICS.
The gun grabbers will be yelling for our guns following this. We need to be the ones that the public views as having common sense and a reasonable approach to these issues. We need to make this a fight of logic, and not of emotion.
Todd
