Hi I just came across this thread. My son is now 8 months old. At his 6 week check they noticed his flat head and favouring one side. He was also referred for hip dysplasia and it was the specialist for that who also noticed he was favouring one side and so referred us for physio. It was very annoying as until that point, it had been noted but no one had referred us, again just suggesting we turning him round in a cot and more tummy time.
Basically the specialist diagnosed torticollis, which is stiff neck (and aparently around 20% of hip dysplasia babies also have it) so he physically couldn't turn his head. I chased and chased for urgent physio appointments but it was this specialist referral from the hospital which got us in quickly. He was nearly 4 months old by then and I had also read up about helmets and stuff so wanted to act fast.
The physio gave us exercises for him, which we did and by our second visit, there was significant improvement and he was voluntarily turning to the other side. The exercise which worked best was to hold him close to my chest (cuddle him) but with his head turned to the harder side and gently stretch it.
He still has quite a flat head even now but has no issue with his neck. the Physio and nurses say that while the fontenal is still open, the head will re-shape but it will take months. but tummy time is key or any seat type thing (Bumboi chair etc) which keeps the weight off their head when awake is good once they can support their head and good neck control.
However, my friend had a baby a few days before me. His head also mis-shaped and whilst she asked about torticollis and helmets, was told that her baby was fine physically and just needed more tummy time etc. His head also is still quite flat and I think it is just something which will be more common these days. I think you just have to look out for the severity of it. More severe cases the front of the opposite side protrudes a bit and apparently eyes become unlevel.
I think helmets seem to be for the really severe cases and looking at my son, it was really flat but have to admit its slowly improving and now the stiff neck is fixed, he'll even sleep with his head on that side. Although now he just rolls onto his side or tummy in the night as he's older so hopefully it will continue to improve.
Hope this helps
Karen