At one month old, a strong preference for one side can sometimes point to tightness in the neck muscles, which a PD can check for pretty quickly. I'd bring it up at your next visit or book one soon, just to rule out anything like torticollis before the habit gets more set. In the meantime, you can try doing tummy time and switching which end of the cot his head faces, so he naturally turns the other way to look at the room. It's worth getting a professional opinion first before trying too many things on your own, since he's still so young.