If you saw one of your Joes doing this, how would you react?

Make fun of him for it. Seriously who wouldn't wear their dress uniform?

This dude is either a poser, or just finished Basic/AIT/OSUT/JROTC, and if he is active duty is on leave before reporting to his first duty station and possibly doing good HRAP, I say this because he only has name tapes, and Army tape on his uniform.. If he was active and already been in a unit he would have his unit patch on his left shoulder

Ok I guestion this picture. Prom really the girls are in cheerleading out fits.
This picture looks more like they were being honored for some school activity.
When I was I. Recruiting if there was a DEP or a JR that completed basic training we held an event with them at the actual school event. Maybe he was just asked to stand in. This picture is suspect

It's 2016, Bury him! Then when he climbs out of the hole that you put him in hit him again! Do you have a need to know? Are you in his chain of command? It appears harmless and like someone else said the ladies are in cheer outfits.

Pull him aside and tell him. Don't go all "Regs lawyer" on him, just mention it to him and make sure he understands that you're telling him because you're WATCHING his back, not trying to stab him in it.
Same team.

And standing At Ease/Attention

I feel a knife hand coming in 3,2,1...

So the person asking the question was in basic with this guy in the photo so its not rotc or jrotc. I hate the what if game or guessing or it could be crap, the question was how do you handle that sittuation when you have a violation of the ucmj. The answer is complex there are different uniform regs for ng and reserve compared to active duty, that said we know its a violation so as my srgnts would say its time for some retraining. This man is soldire at the core why the need to worry about kid gloves tell him he violated and why then make sure he wont forget it further actions taken the time. Uniform regs are in place for a reason its about discipline and character you wear dress uniform for formal occasions non military related or military related its not our job to question regs we follow regs details matter

Reminds me of the time when I saw 2 Privates standing in line to get into a dance club in their BDUs I went over to them and told them the regulations and then asked what unit they were from and got their 1SG's and Captain's phone numbers. They quickly left at that point! Hahahahahaha!

He might be in the National Guard...