Please don't take this the wrong way and I am not insulting your doc BUT if it is a PWS it will not fade on it's own. It will appear to become "darker" or "lighter" depending on temperature and his well being. If he is happy or mad.
Generally speaking, if the stain is causing the Doc to reccommend eye doctor visits, than they usually will also reccommend an MRI.
I just don't want you to get your hopes up that it will fade on its own.Stork bites can fade on their own.
The forehead tends to respond well to treatment.
My daughters scalp was treated because she lost all her newborn hair and it was possible to treat. If she did not lose her newborn hair, tx of her scalp would not have been an option at that point.
PWS is not commonly seen and many medical professionals have never seen one. It is not covered in Nursing SChool.
Just the common "birthmarks" are mentioned and mentioned very briefly.