It depends on what you're son's interested in! If he's into Marvel/DC, you can go for a superhero costume party theme. If he's interested in science, you can have a futuristic/galactic theme. The important thing to keep in mind is that it's his day and he should have fun with a them he and his friends can truly enjoy! :)
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You can always go for his favorite character or if he doesn't have one, go for a more generic theme like safari, sports, music, etc. I think what will give oomph to that is the fact that it's your son's interest.