Okay, so he is technically middle aged in soul, but actually he is still a kid both in body and head, is how I would sum it up.
Just cuz he said he will go to the battlefield doesn't mean he is smart/grown up. Getting excited to write a letter to a 10~ year old girl is being childish. Wanting to...
The Duke and his son should have been slain then and there... but then the Main protagonists would have died too so I guess this wasn't an option.... *sighs*