My father drinks only coca-cola. nothing else. no water, no tea or coffee, no other drinks. this is very disconcerting to me. addicted? how do i help?

Read and tell: Look up the negative effects of this on the internet and express your personal concern for his welfare, outlining the negative things you have read. His response will depend in part in the manner in which you approach this. If you are accusatory he may reject your advice. If informative and concerned, he might listen. But he is an adult - some people don't learn.