Advice from a headteacher.
First, speak to your child's class teacher. There are all sorts of reasons why he may have got GDS in some things and not Maths. Some schools only use tests, perhaps he messed up on the day. Maybe he's one mark off GDS and there's not too much to worry about. Maybe he has a very specific confusion with, say, fractions, which can be worked on short-term.
If teacher feels that he is less strong on Maths than Literacy, then the right tutor can make all the difference. It's probably short-term (1 to 2 years) and based on carefully assessing and gap-filling backwards. If he's not getting long division in Y5, for example, teaching him it again just masks the problem; analysing why he's not getting it (concept of division? place value? remainders?) will fix the problem ... the aim of the game is to enable him to be ready to understand the learning in class, rather than needing tutoring forever. If you can get hold of a copy of the long-term planning, then pre-teaching key concepts before the teacher does is very effective. Be very very wary of tutors who are going to deliver a "curriculum" - it only works when it is bespoke.
Finally, if this is a one-off, maybe the class teacher last year isn't very good at stretching children in Maths, and this is reflected in results. It happens!
Your child is very bright, and sometimes underperformance is about confidence, particularly in Maths, where willingness to have a go and solve novel problems using known skills is the difference between GDS and EXS. It would be worth investing in a set of CGP books (start with the greater depth book for last year and (hopefully) rattle through it and fill the gaps, then expected for next year, then greater depth for next year. Over the course of the whole year.