I agree with everyone on here. You need to file a claim because deer heads ARE worth money and this guy stole yours. I would call around to various taxidermists and see how much they cost and whoever is the highest, that's what I'd go for in this guy.
It's obvious this guy is a peice of work. You need to tell everyone in your town about this experience and he obviously needs to GO. When I take my deer to the processor it isn't a WEEK later that he's calling and already done with ALL the meat. Many times it's 2-3 days later.
Here's another idea that I would do... write your local editor in the newspaper. Tell it like it is and name this guys business in town. Now, you'll have to give YOUR name too because they won't run the letter without it, but that's a small price to pay PLUS you're going to have the majority on your side after the word gets out.
Good luck,,, and sorry for your son
...and on that day when my strength is failing, the end draws near and my times has come, still my soul will sing Your praise unending, ten thousand years and then forevermore!