Well he is a human being. Would be weird if he was perfect and fulfilled every expectation as a lord or a main character. Panicking in front of others is something that happens to a lot of people regardless of age and even people who may be able to control their expression while doing that, can visually panic sometimes, especially if it's so unexpected it catches them off-guard
yeah, that's what I was thinking too. I know that he's only like 12 or something but this guy is a reincarnator so he should be more mature than his physical age would imply. I don't recall how old he got to in his past life but he shouldn't be screaming and flailing at the drop of a hat.. especially as the son of a Lord. I'm sure he was raised with certain etiquette standards and taught how to behave and how to carry himself....
But then again, he may be a reincarnator but I don't remember his past life as being a noble or warrior or ever been in war before. Knowledge fr books and such is different from actually leading wars or governing cities. Also, even if he has past memories, he's still 12-13 so it makes sense. After the reasonable initial panic, he calms down and starts thinking again. I think he's portrayed well by author.

I really hate how quick Ars is to visually panic and scream like the sky is falling.