i'm not sure but i feel like it's probably a child that he couldn't save on the battlefield?
maybe a relative? i think they looked quite alike so maybe his little brother or nephew?
He mentioned earlier when talking to the other soldiers that he had a wife and kid that left him. It could have been fake backstory stuff, but since Bard is currently in his mid-thirties, he'd likely have his own family at his age.
I'm fully betting it's his son.
I remembered Bard mentioning something about his nephew looking like Ciel in the anime or whatsoever. I just remember it, so maybe that's who he is?
who is the boy that cook remembered while looking ar ciel