If he eats or drinks something sugary or starchy more than three times a day he will get decay .
It doesn’t matter what he eats or drinks at meal times it’s what he eats and drinks in between that’s the problem. Each time he takes a sip or bite of a sweet or starchy food the acid attacks his teeth for an hour afterwards. If he eats or drinks sugary/starchy stuff just before bed , even if he brushes his teeth afterwards , the acid attacks are even longer because less saliva is produced overnight.
That’s not just sweets, chocolate , biscuits , cakes but fruit juice, smoothies , protein shakes , energy drinks , yoghurt. , raisins , granola bars , fruit winders etc all of which are as bad as fizzy pop and sweets for teeth .
Unrefined sugars eg honey , agave syrup etc are just as damaging for teeth as white sugar.
Is he a grazer ? This is the worst thing for teeth as it means they are constantly under attack.
Does he use a fluoride contains toothpaste?
Diet,diet , diet is the problem. He already has had fillings and now there is decay under those fillings . He will lose teeth soon if he doesn’t change his diet now.