@HDMI1 I was wondering about that too, first, but the 3rd page here (which wasn't on pixiv, it was a picture posted on twitter another time, but I thought it's tematically fitting here, so I added it here.)
So, the 3rd picture was originally titled "parent and child". So, yeah, I'm assuming Hiroto is Souta's father.
He did appeared before too, and, there's only one character like him on the character introductions extra pages, so there isn't another guy like him, like a brother - although sometimes he appears more like that. but since this page states He's Souta's father, so, yeah, I think he's that.
He do looks quite young, but again, we don't know how old he was when... so, yeah, he might get ara ara'd too and... might became a father relatively young? who knows.