It wasn’t within a few months the thing that you have to keep in mind is that Viltrumites have the exact same range of emotions as humans. But, what with the way their society Works, they are conditioned from a young age to only acknowledge ruthlessness and loyalty to the Empire. Nothing else.

We see in flashbacks that Nolan was still a little bit emotionally closed off when it came to Mark when he was a child, but he eventually started opening up and acknowledging the feelings that he had never been allowed to experience among his own people.

And when he and whenever he’s confronted about caring about something that does not fall in line with what he has been taught his entire life, he doesn’t even know how to vocalize it because it’s a feeling so alien to him.