My child has a PWS on his face, it is surely PWS. And it also becomes white when I press it. I think this is normal, because capilares on PWS are wider than normally, and so they hold more blood. When one presses the capilary is squeezed and for a moment with no blood, thats why white. As soon as you release the pressing the blood comes back again to its position and the PWS appears again.
I hope your son does not have KTS, and that further examination will show only that, nothing else.