My son was the same way. Really encourage him to turn his head to the other side. My son's head is still flat and probably won't curve out all the way, but he didn't need a helmet. It did shape itself out a lot as he got older, just not all the way. He'll just never be able to shave his head, LOL
ANYWAY, we would put his bouncy seat or whatever turned to the side, so if he wanted to check out what I was doing, he'd have to turn his head to the side he did NOT prefer. We also massaged his neck and shoulder muscles to help loosen up the tightness so it was easier for him to turn his head the other way. As soon as he can hold his head up, get a Bumbo... this helped a LOT because he was up and able to look around better than in anything else! When my son was sleeping, it was a lost cause... he STILL turns his head that one way!