Hi. I have Syringomyelia and I am sorry I have never heard of the concerns on sunscreen before, but I will certainly look into it.
If you are looking for some great products to help him cool and not be soaking... ( the evaporation ones drive me NUTS) I have ordered vests and wrist/ankle bands from :
http://www.polarsoftice.com/koolmax.html
they have saved my life and from overheating as they use ice packs specially sized and shaped for each product. I hope this helps and blessings to you, your family and of course your son.