Reread his question. You seem to have ignored all but the first sentence.
If his gun is held in his back pocket, and the sights and grip are visible, it is not hidden from common observation. It would be considered open carry in Virginia for the same reason a gun with its butt and rear sights exposed and holstered in an IWB is. (See Virginia Tuck) Other states are different. This is not a State forum where State specific information belongs. That's why there are 50 subforums on the main page.
Not hidden from common observation? Hidden from common observation in LEO terminology means that it must be visible from 3 sides and readily identifiable as a firearm. Engagements happen face to face, not face to back; simply placing a firearm beyond the 180 degrees of the forward view of the body may constitute concealment for that reason alone. At least that's what I was taught as a MP and when I had to learn Georgia's CCW laws. If it's in a back pocket it is only visible from 1 side, not necessarily identifiable as a firearm (cannot discern it from a BB gun or toy can you), and thus the intent to conceal may exist in the eyes of the law. Likely, most State's do not have coverage for this type of carry in their statutes....it means it's likely to be clear as mud on this issue.
That said, one needs to look at other factors of carrying in such a manner in the back pocket. It's VERY dangerous and VERY foolish, I've tried it myself and went to a front pocket carry with a Ruger LCP. Here's 10 interesting things to consider:
1. Your eyes are in the front of your head, you cannot see a threat behind you that may attempt to take your partially visible firearm.
2. Your arms tend to reach forward rather than backward, this makes #3 a reality.
3. Firearm retention from a threat is an impossibility....say it again...retention is an impossibility!
4. Firearm retention from an accidental drop is difficult unless you go fully concealed in the pocket, so the material of the pocket acts as a retention device.
5. The type, size, caliber, and capacity of any firearm carried in the pocked becomes extremely limited. Sometimes smaller is better.
6. Pocket holsters are designed with the intent to conceal, use one and have to go to court and the description of that equipment may reinforce "intent to conceal" even if that was not the intent.
7. Sitting on a toilet can become hazardous...not just because of a potential firearm drop, but because you could dislodge your firearm and lose it....this has actually happened in a Wally World. Person's firearm fell out of his pocket and was left lying on the bathroom floor, fortunately an employee found it and turned it in.
8. It's going to print and print bad, not to mention it eventually wears a hole in your pocket like a can of chew will.
9. Ever try to sit down with a firearm in your pocket? It's not very comfortable...think George Costanza and the wallet issue.
10. Ever try to draw your firearm from a back pocket while seated? It's simply not going to happen very fast if at all.
And by the way folks, front pocket carry has some of these same issues...especially those regarding a seated position. My wife had to find this out the hard way after I had tried to educate her on the issues I had experienced. I eventually wound up having to buy her a Crossbreed Supertuck for her Ruger LCP.