The Munsters' car shot flames out the tailpipes, and that was way cool. I would leave it.
It is possible with several combinations... leaky injectors... bad valve seat... bad timing... and bad coil pack where it doesn't burn anything and the cat converter ignite that fume all the way out the tailpipe at highway speed.
You can verify this by following your son while he is driving on highway... be sure to have someone in passenger seat to take a picture or video of it...