In the UK we would not use 'in' as the preposition. Does he have to wear a uniform to school? Does he have to wear a uniform at school?

Yes, you would normally omit "the". "in school" means "the state of being in a school". There are contexts where "the" could be used here, for example, if you had brought up the subject of a new school, and asked if had a different uniform policy than the child's old school. "The school" would mean the new school. But that would not be the common use. By the way, in your sentence, you could also say "at school"; that is just as common. It emphasizes the physical location a little more, but your sentence would mean the same thing.

Also, you might use "the school" if you were asking questions about a new school (again, this would be a context where you were talking about the particular school's and its policy, and not the child's current situation in the school). The context here matters.