Regis hates this situation. Cor brought back an MT, and when he found out MT's are children, and being experimented on, Regis decides to let Cor take care of the boy as if he were his own, as it's obvious Cor has become attached to the boy as well (the kid is most comfortable around Cor and listens to him)

But Regis now has to sit here and watch his council fight over this child who's obviously terrified and just wants to go home

So the council overrides Regis's decision and forces Prompto to prove his loyalty lies with Lucis: he's outfitted with a shock collar and if he truly is now loyal to Lucis, he would shock himself to death

So here is Regis watching this scared teenager crying and trying to press this button and panicking it wasn't working

But there were no batteries in the collar, the council reveals. They wanted to make sure Prompto would not be a threat. And Prompto passed the test but Regis is furious as he runs from his throne to console him and tell him it's okay but Prompto is still crying and asking why the button isn't working