I went to the local Penney's for some derma blend as well. They did help me find the right shade for my face (I have rosacea, not a PWS). However, even as an adult, I found it a pain having to apply the makeup AND the powder.
Maybe take him around to some other makeup counters too and see if you can find something that matches really well and won't be too much for him to wear (like, no setting powder).
On the other hand, I came home and put that derma blend all over my face! When I looked back up, my lips were gone!!!! That stuff will cover up anything!