You say he used to play number games in the car when he was younger, as an example of how much he used to enjoy maths.
sometimes children do well in a subject playing games with their parents, and learning at home. They learn a lot and enjoy it and show promise. At school they are ahead of theor peers and enjoy demonstrating their ready made knowledge and skills.
These children are not learning in lessons, they are simply practicing what they already know.
Sooner or later, the class moves onto something the parents HAVEN'T taught first. The child can then come a cropper, because although he may be good at learning 1:1 from parents, he may not have ever really learnt to learn in class situations in lessons.
In these circumastances the child becomes anxious, and drops down the class in subjects they previously excelled in.
All children of involved, caring, parents, who stimulte their children intellectually, are likely to find themselves in this situation sooner or later. Generally , the earlier the better, so they mess up in primary, not secondary.
If this is what has happened with your son, it is just a blip, at the transition between what he had learnt from you, and the leaning habits he now has to pick up at school. It might be worth spending some time setting him up to practice working independantly, with computer programmes or text books. This will help prepare him for next year.
I'm not saying son't employ a tutor, just that is a complwtly separate issue.
I hope you have a lovely summer holiday, and he starts next year feeling positive and empowered, however you end up doing ti.