He needs to see a dentist if ever there is something that doesn't seem right. Although I'm unsure I'd equate drinking cans of soda with black lines. We all surely want to keep our natural teeth for life, if possible.
He may want to ease off on the cola anyway. Unless drinking decaffeinated, then he could be taking in an overdose. I know at one stage of my life I was drinking too much cola and it started to give me palpitations and skipped beats ! When I found out the cause I eased off it and the symptoms went away.
Personally I am incensed that the government permits foodstuffs on the market as acidic as canned drinks seem to be. We now know, too late, that even normal consumption can, over time, make tooth enamel thin. Surely manufacturers can reproduce the flavour without the Ph level ? It is just gross negligence on their part, and on the part of the food standards authorities.