If you can afford it maybe look into a private tutor who is a school teacher and can set him tasks at the higher grades curriculum. My best friend is a primary school teacher and has done this for a few families where she lives as the rule is that you can't tutor kids at your school because it could be seen as a conflict of interest.

She writes up homework tasks that fit in with the higher classes curriculum but doesn't cover the whole thing as otherwise your DS will get bored next year when he does that stuff in class - or they may be able to set harder tasks on the topics he is currently studying at school.

My best friend doesn't charge too much for her tutoring services but as the teacher sets their own rate there are some teachers out there that charge too much. Could you look into something like this?