I get that you notice this, but does anyone else see it as a problem?
You said "his head was narrow due to being born early" - says who? Maybe he just HAS a narrow head? I don't think flat spots change over time. I think either you have them, or you don't.
If you didn't notice this when he was younger, maybe this is just a "phase" - seriously. My children's faces have changed SO much since they were babies, just like their bodies. Sometimes they do look so different than just a year ago, I can hardly believe I haven't noticed the change until I look back at pictures.
I too have noticed faces (and bodies) going through phases of elongating, then they seem to stop elongating, but start to grow wider, thicker, chubbier. Then as soon as I notice, it seems another growth spurt hits and we're back to UP growing and not OUT.
My 1st child had a cute, cherubic face at age 4. When I look now at her 1st grade picture, I can't believe how long her face is. However, she's also grown UP so much too.
Unless you or your child feels he is deformed...I'm not sure how severe you think the asymmetry in his face is...I would just wait to see what happens next. He's only 6!
If you decide you want to take him for an evaluation by a specialist, just please DON'T talk in front of him. Come up with some reason he needs to see the doctor. Don't make him feel self conscious...especially if it's something that you decide is too minor, costly or invasive to risk changing now.
NOTE: I have heard from others that "flat heads" that are minor really are only aesthetic when it comes to head shape. That when kids have hair, the hair covers the "flatness" - I guess very few people have perfectly round heads. I know several children who have worn helmets. In a couple of cases, there was a severe problem that definitely was causing facial distortions already at a young age, the other two, the flatness was like a huge dent and 1 other was a back of the head that prevented her from rolling on the back of her head because it was so indented.
I hope this helps you.