The M-9 is the official military issue sidearm for all branches of the military. But almost all branches of the military have units that have funds to spend as they wish. It is not uncommon for them to use non-standard firearms on a limited basis.
If he served in the middle east recently it is also common for soldiers to use guns they find, or pick up, especially handguns. They are not allowed to bring them home and they are given to someone else when thier deployment is over. This may not be by the book, but many officers look the other way and allow it. This could be the case here. I've several friends and 1 relative who did this druing their time over there. Seen photos of US soldiers with almost every imaginable handgun on them.