Well, I don’t know how the National Guard works (I’m Active Duty), but as I understand it, if you want to join the Army and you’re from another branch, you have to attend the 4-week Warrior Transition Course first. I would imagine that would apply for prior service - maybe. I think if you leave the Army for more than five years, you have to re-do Basic Training. Which is understandable - but that would suck.
Oh, and my point was, that it is surprising he would work with the unit right away, without any Army indoctrination. And yes, the US ARMY goes on your left side, over your heart. Unit patch on left shoulder, flag on right. Putting your patch under the flag would be a bad idea. :-) That’s the one part of your uniform where everyone wants to put a patch, but a lot of people can’t. Putting it there makes it your combat patch.