I know of two kids with a flat spot. One I don't see anymore (we went to the same infant massage class) but he started wearing the corrective helmet at six months and didn't mind it at all. His mom was worried initially but once they started using it, she said it was a breeze. By contrast another mom I know was offered the helmet at 6 months and she decided against it. Now her LO's head is evening out but now at 11 months it's still noticeable. In all fairness, he's a big boy and slept in a Velcro swaddle until recently so he didn't roll over until he was 6 or 7 months. He also didn't enjoy tummy time do he was on his back for a lot of the time.
I'm not trying to scare you into using the helmet but it's the right time to consider it. In your shoes, I would make an appt to discuss it. The dr usually takes measurements to assess the flatness and can quickly determine if the flat spot LO has is likely to correct easily on its own. Since your LO spends a lot of time off his back, I doubt you'd need the helmet but I would look into it just in case. Hth