Wow, so much negativity from the posters in this thread.
Apparently, from the link posted by SBELL, Disney, along with a lot of other corporations and government entities issue the letters if requested. The OP isn't asking for anything that doesn't already have a history of being done if requested.
So what is the big deal and what's so wrong with the OP asking for one for her son?
Don't like or agree with Boy Scouts? You wouldn't personally do it? Fine, you're entitled to your opinion but for people involved with scouting attaining Eagle Rank is a huge deal and it comes hard-earned. If OP wants to celebrate that for her son w/ a letter from a place where they've made many family memories, more power to them. It's a keepsake that he can keep with his other scouting mementos and look at years from now.
If you've been married 50 years and you (or someone in your family) sends in a copy of your marriage certificate, the President of the United States will send you a congratulatory letter (as least they used to do this). We did that for my grandparents on their 50th. Sure, it was a form letter and I doubt the President himself actually signed it, but my grandparents still got a kick out of it when they received it.
What's so wrong with doing something a little special to make someone feel good?
Congrats to your son OP!