Welp, like brother, like sister, they're about to get fired for not knowing better.
@Bekka-m : He had like 3+ chances to not be an idiot and he fucked it up.
The king is clearly ok with some amount of informalities from the knights with how they interacted with Nina in ch 40, so the kid just not being respectful initially is one thing.
At his initial appearance, when he went 'who're you?' + 'leave now or we'll get into trouble'
If he stop to think for a moment, he should've realized that whoever these people are, his boss isn't having any issue with them.
Then when Leo kicked him and apologize.
Then one last time when Leo went all the way to kneeling to apologize once more.
As Nina pointed out, it doesn't really matter who/what is going on, when your boss is kneeling you shouldn't be standing.