I like to think the Crystal gave the royal line a multitude of special abilities to go with the magic and one was having visions. Let's say leading up to the roadtrip, Regis has visions of Prompto as a child in niflheim and in the future on their journey
+The first time Regis sees Prompto experimented on as a baby and smuggled out, but it must have been a dream because here's Prompto trying to do math homework with Noctis in the living room
++The secomd time Regis gets glimpses of Prompto in a tank, being punished and held down by MT's but that isn't possible because Prompto just arrived to take Noctis to the arcade
+++Another time Regis sees Prompto being injected with daemon blood, running through the snowy tundras of Gralea and trying to evade a giant mechanical beast but that isn't right because he's here posing for photos at his and Noctis's graduation
++++Just before Noctis and co leave for the big roadtrip, Regis's final vision is of Prompto in Ardyn's clutches and it's the worst vision of all because Ardyn is toying with him, abusing him, etc and Prompto looks dead already and Regis finally gets a glimpse of that barcode on all MT's but that isn't right because he's climbing up the stairs to tell Noctis it's time to go on their trip
++++++Each time Noctis keeps asking Regis why he looks at Prompto funny sometimes but Regis can't bring himself to tell him
+++++++Regis gives Prompto a hug almost as if he never wants to let go and he tells him to be strong and that he's one of the bravest people he has ever known
Gen Regis + Prompto, Regis has visions of Prompto's past/future each time he sees him
+The first time Regis sees Prompto experimented on as a baby and smuggled out, but it must have been a dream because here's Prompto trying to do math homework with Noctis in the living room
++The secomd time Regis gets glimpses of Prompto in a tank, being punished and held down by MT's but that isn't possible because Prompto just arrived to take Noctis to the arcade
+++Another time Regis sees Prompto being injected with daemon blood, running through the snowy tundras of Gralea and trying to evade a giant mechanical beast but that isn't right because he's here posing for photos at his and Noctis's graduation
++++Just before Noctis and co leave for the big roadtrip, Regis's final vision is of Prompto in Ardyn's clutches and it's the worst vision of all because Ardyn is toying with him, abusing him, etc and Prompto looks dead already and Regis finally gets a glimpse of that barcode on all MT's but that isn't right because he's climbing up the stairs to tell Noctis it's time to go on their trip
++++++Each time Noctis keeps asking Regis why he looks at Prompto funny sometimes but Regis can't bring himself to tell him
+++++++Regis gives Prompto a hug almost as if he never wants to let go and he tells him to be strong and that he's one of the bravest people he has ever known