Amey,
With my boys, I do set aside their school books, so they don't quickly plow through them.

Otherwise, I'd be continually having to find new books for school rather than using what I'd planned (which gets very expensive).

I also want my boys to understand that their books for school have a different purpose than their free-reading books. We read our books for school in little bite-sized pieces and dig into them more deeply, while their free-reading books are theirs to plow through as quickly as they desire.
I agree it is wonderful that your son is excited about reading, however you'll find he'll be even more excited to see what happens next if he has to wait and come back the next day before reading the next section. Once my kiddos finish a book for school, then they get to have it up in their room for free-time reading.
Blessings,
Carrie