I've been trying to find decoration ideas etc and drawn a blank so far. Hopefully someone may still see your question who has a recipe, but you probably don't have much time. So you could have a look here - under "Stop Signs and Picks" (right hand side) you will find little plastic decorations like road signs and traffic lights - not what you are looking for I know, but the site is full of ideas related to vehicles etc and you might find something that appeals to your son? Otherwise maybe make an ordinary cake with icing/frosting in red/orange/yellow, or even a pattern of 3 circles in those colours if you are able to do that.
Hope it goes well
Hope it goes well