AHHHHHHH!!!!!!!!!! Oh my GOD! What a nightmare! I had a similar situation happen...as a grown up too!
My ex boyfriend thought he was helping me...I asked him to put my splint in a cup of water with a Polident tab...he used almost boiling water. DUH! It sat there for 20 min or so...I found it melted to the bottom of the cup....yep...$400 down the drain!
At least your son is just being a kid!
Cheer up, at least it didn't break while he was wearing it!