Welcome to the group.
I have PWS on the right side of my head that goes into the back of the head on the crown. I have pictures you can look at of when I was a child/infant and pictures of what it looks like know when I had water in my hair so the PWS would show very well. You can see all of these pictures on my website at www.hankspws.com
Ok, from what you have said so far it sounds like your doctor is on the right track. I would diffently recommend the MRI. Make sure though when your doctor requests it they have it with and with out contrast. The reason why I say this is because being that your son has PWS on the head he does have a chance of having Sturge Weber Syndrome (which I have) that could lead to other issues down the road that you should be aware of. I have links on my website about SWS if you are intersted you can also find a lot of those links on this group Sturge Weber Communty is VBF's branch of SWS.
As for the laser treatment. Early intervention should be the way most doctors should recommend but some do not. I think sooner the better because you would rather get the PWS reduced in size before it effects your son emoinanally as he gets older. I have gone thru that trust me you do not want your son to go thru it.
The V-Beam Pulse Dye Laser is one of the most common lasers used today. It is safe for infants, effective depending on the PWS, and treatments do not take very long at all. I like this machine because of the cooling device and it does not scar at all unless used by a doctor that does not know what they are doing. Your doctor you have mentioned already sounds like they know what steps to take. Or as Missy mentioned there are some good doctors in LA in the resource section above.
Hope this helps.