First of all, if you are not comfortable with what your doctor has told you, get a second, third, and fourth opinion! You do not have to go by what one person says, especially if you question it AND it is in regards to your child.
Our doctor (Dr. Buckmiller at Arkansas Children's Hospital) told us that the PWS typically responds better to treatment when it is done as early as possible. She started our son at 8 months. He's had two treatments and we've seen tremendous improvement already.
Our son's PWS is also on his face -left cheek, and we had been concerned about cosmetic stuff. He was also checked for SWS and will continue to receive annual check-ups with the eye doctor to make sure the PWS is not causing any problems.
We were told that treatment was medically necessary (not just cosmetic) because it was on his face and could cause medical problems as he ages, i.e. stippling, bleeding, etc. Our insurance company even thinks it is necessary (no appeals or begging!) because it is on his face.
Again, if you aren't sure about what your doctor told you, get another opinion. It won't hurt the doctor's feelings and will be better for you in the long run, even if you decide to do what your doctor has already suggested.