I checked this question for you. The only correct answer is ‘Is it blue?’- as shown in COAsT.
The reason for this is, because it is a flow diagram, the wording in the missing box needs to be exactly the same as the wording on the other side – which is ‘Is it blue?’. This will then lead up to the next answer choice in the flow diagram.
The answer that your son gave of ‘BLUE BOXES’ is incorrect. Have a go at working through the flow diagram with the correct answer ‘Is it blue?’ in to see how it works.
I hope that helps you 🙂