Tooth decay is caused by too much lacto bacteria in the mouth.The bacteria feed off of sugar,one of the worst sugars for tooth decay is juice.Here is an easy solution to help.after he drinks juice or has a high carb meal make sure he rinses with water,that will solve alot of the problem.make sure he rinses with water before bed because thats when your body stops making alot of natural saliva and when most tooth damage is done.
Hope this helps