Is he an idiot? The reason he couldn't go was because he couldn't stomach watching people die, not because he wasn't skilled with a blade. Training is pointless if he still can't order his troops to fight and kill, and deal with the consequences. The normal way for him to develop that skill would have been for his father to have him accompany him on campaigns and be there while judging criminals and such, instead of keeping him locked up in the manor where he is isolated from the harsh reality of being a feudal lord. But his father is also an idiot, so instead he kept him home even though he knows he is sick, there's a war starting, and sooner rather than later Ars will have to take up command.