Have him look up the effects and such. Tell him about the risks of obesity and diabetes. My family was exactly like that, and all they would drink was soda until my Nana died of it. Have him understand the serious risks.
It isnt just about weight dangers. If the soda is sweetened with high fructose corn syrup or corn sugar... You can pretty much count on him eventually becoming diabetic. High fructose corn syrup and corn sugars are a federally approved for human consumption, poison. His weight will go up because those toxic sugars will start to prevent his pancreas from releasing the proper amount of insulin to regulate his blood sugar. He will eventually have to give himself shots of insulin for the rest of his life. Unfortunately, this is not a joke.
Why don't you do some research on the sugar content in soda drinks & show it to him. 1 litre of coke contains something like 14 spoons of sugar - that would make it 3 1/2 spoons per glass, average size. If you do the research, he's gotta believe it .... surely. Otherwise, I would suggest you ask someone in the medical profession, if you can, to speak to him. I hope it works
Soda isn't usually something you should have everyday . It isn't the best for your body. It is full of sugary and if have too much it can cause you to have diabetes. I had a bad case of drinking soda everyday. I love cola and that is all that I would have through out the week when ever I was able to drink soda but then I realized I need to cut low so I took almost a week off of drinking soda and I felt really healthy. Water is usually the best to help take care of your body and to keep it healthy !