Since you have nearly a month, is it an option to take him to the local fabric store and let him look through the pattern books? That might be an inspiration.
What about a pirate? This occurs as we have homecoming this week and both kids put various pirate costumes together for Thursday when they dress like their opponent!
Today was favorite Hollywood personality, so what about a movie character? At 10 their options are not as wide perhaps as a highschooler but as an example DS went as one of the Blues Brothers. A couple of years ago he dressed up like Dick Tracy.
Our favorite place to scrounge for costume parts is the local charity store (such as Goodwill, etc.)
What is his favorite book character right now?
Deb