Hello and thank you for your question:
Burning rubber is a new one for me. Everyone has a different opinion on smells so I may conclude that it smells like something else. A few things I can think of
or atkins diet
(low carb high protein) can lead to acetone breath which is very strong
-gingivitis/periodontits which to me smells like rotten raw chicken
-Coffee/tobacco that can have a pungeant smell
-tonsiliths (hard mineral deposites in the tonsils) these have a very strong odor
-a diet high in garlic, cumen, curry, onions tend to give off a strong odor
Most likely it's due to poor oral hygiene
and needs to be addressed.
A lot of bacteria that causes halitosis
or bad breath
is on the top surface of the tongue. Have him brush his tongue in a back to forward fashion. Floss, brush and rinse with mouth wash daily. If the odor persists, suggest an evaluation with ENT for tonsiliths and to have diabetes ruled out with internal medicine.
Hope this helps.
Best regards and good health