Thanks for coming back to explain. Really, I'd not go to all the trouble and expense to change his first name for that- just continue to use his middle name- even if his official name is the first ... and long. This is very common, in the UK too. At the end of his schooling, you could always ask the school to amend any papers with both his names. I have no idea if the above scenario would be sufficient for a change.
As far as I know you are free to use any of your given names. I know several people who go by their second or third names without a problem. One can have a note in a Swiss passport indicating by which name one wants to be called, but this is not mandatory and on simple ID-cards this field doesn't even exist.
I have that same issue. Well, I have first name and last name issues. But the first name/middle name thing isn't really a problem. Whenever I fill out a form I just underline the middle name and in most cases things are addressed to me by my middle name.
Honestly, if it's ONLY one first name, I wouldn't bother changing it here.They get their knickers in a twist when you talk about name changes. LOL. And since anyone can use their middle name they probably won't accept it as a valid reason.
Wait till you go back to Texas or if it really bothers your child when he grows up then he can change it. All of us siblings have this issue and when my younger brother became an adult he decided he preferred to use his first name.