Hun, first of all, regardless of his head shape, he's a cutie pie, he's loved, he's healthy so - rounder head or not, the important things don't change.
Second, do not beat yourself up. This is not your fault. It already happens a lot especially with the sleeping on the back. The issue here is not you. I actually have this talk all the time with the physiotherapist and osteopathist... the doctors *always* say, it's gonna get better, it'll fix itself when they sit up. BUT that's not how it works. I've actually seen very few kids where it *corrects* itself on its own. Plagio doesn't because, if your kid had an issue of laying to one side, it doesn't magically go away when they sit up and then they'll still sleep 8+ hours on that side.
Brachy, same thing. By the time they sit up, they're 6-8 months. The bones are the most malleable before 4 months. By the time they sit up, the head doesn't grow as much as fast. So if they sleep on the back of their heads still... well! And some kids don't have a lot of growth spurs, and what if it happens in a bouts where they are more laying? I wish doctors would stop saying *it'll fix itself* and then parents feel guilty when it doesn't... Because hun, you love him, you care for him well, and you went above and beyond. So please, do not feel guilty.
Plus, we all have different head shapes! He doesn't look weird or funky. I see some kiddos with a head like WAY rounded and sticking out than my kids and yet they are NOWHERE near a yellow on brachy and newborn heads always stick out a little more
because nothing is fused.
SO PLEASE, no guilt! You're an awesome mama doing an awesome job!