Second opinion - always!
Aside from that, use your gut as a mom. You probably have a feeling as to whether your LOs head really is that flat.
If this is something that will benefit your baby, then do it. A few months with a helmet is not a big deal IMO. There are far worse things your baby can have - that's how I see it. Who cares if people stare. The hell with them.
Also, try using this...
Use it for a bit and see if it helps any. Basically use it at all times except when your LO is sleeping. In the car seat, in the bassinet, during play time on his back...use it ALL the time. Hopefully it can help shape his head back to a rounder shape.