As a father of two sons that are also Terraria fans, I can wholly understand that feeling. I wish I could do more - but all I can do at this moment is share what I'm hearing back from the tech side while I push and push for any possible fixes.
I know that doesn't make it hurt less for your son - and I know the CS guys mean well when they are trying to help as well. It kills me to say anything other than "here is the magic solution". I can quite easily empathize with your plight - I know what my sons' Terraria files mean to them.
This was also a big impetus for the push for a backup system that everyone will have coming their way soon (some sooner than others)....to try and help lessen the blow from situations like this.
Regardless, this particular issue is now on top of my pile of issues to pursue - for what it's worth.
Hang in there and maybe that solution will come.