Getting kicked out (meaning dishonorable discharge? discharge under other than honorable conditions? bad conduct?) for a single DUI seems extreme to me given what I've seen of military discipline, especially for alcohol-related offenses.
I also thought it was extreme if that is all he has done. My brother is an alcoholic and was in the military for 22 years and then seconded from them to do work. He did some awful things they covered up and let go, including crashing a tank while drunk into and over a line of civilian cars in Germany and having an inappropriate relationship with the daughter of an ambassador he was supposed to be protecting. I think way more will be revealed.