Thanks for all the suggestions. I did think about the Scalextric but discovered the boys had a Micro version which was a bit rubbish and I don't think they've got space for the full sized one - plus they cost £130+ for a very basic set.
The got Nerfs last year and the dog keeps trying to eat the pellets.
And they already have a go kart and remote control car. That's the problem with him having a brother six years older, it means that he's already inherited a lot of toys.
So as he has a huge crate load of Hot Wheels I decided to get him a Hot Wheels triple track, plus the book and also found a Disney Cars cereal bowl and plate set.