The pediatrician says our baby has an asymmetrical head and that we can select to have him wear a special helmet. Is this really necessary?

Maybe. Some asymmetry improves on its own with simple efforts like changing sleep position, neck stretches. Occasionally, physical therapy to improve a neck issue is also needed. Sometimes, in spite of these the shape gradually worsens and helmets can provide a better cosmetic outcome. When started too late, it is less likely to help.Those who have examined the kid are in the best position to tell.
Unknown. This really depends on the child's age is he a normal infant or does he have a condition that would constrain his movements like muscular problems were hypotonia. I very rarely recommend a helmet because they're expensive not usually covered and most children will have a head that goes back to normal over the first year of life. More tmmy time and make your pediatrician check his neck to make sure.
Asymmetrical head. We can not double guess your pediatrician who has examined the baby and the helmet is only to mold the head into a symmery as babies head will be growing as baby grows.If you have concern, get a second opinion from anther pediatrician.