Heathfield's suggestion sums it up - Cola rots your teeth (due to sugar and acid - the latter of which is in diet versions too), leeches the calcium out of your bones and contains six teaspoons per can - so your son is having up to 48 spoons of sugar a day, just in his cola.
It also contains caffeine, which makes you jittery and edgy - and is addictive - which could be why he drinks so much. He would probably be best cutting down by a couple of cans every few days so he's not tempted to keep glugging.
Oh, and caffeine is also a diruetic - so the more you drink, the more thirsty you get and the more you need to keep drinking (when your body is actually crying out for water).
Hope this helps