How bad is this? Chalky teeth MIH

[RosieChardonnay]

The dentist diagnosed my son with chalky teeth (mih) last October. He has a checkup next week and I just noticed this on his back tooth. I am really worried as I know chalky teeth are hard to fill and can decay quickly. How bad does this look? We are so careful with his teeth, he carefully brushes twice a day and he has sugar as a treat occasionally and only directly after a meal. I try to avoid snacks and never allow sugary snacks.

My daughter has reflux which is damaging her teeth. One thing her dentist has suggested is to put toothpaste on her molars before bed. This is after brushing them at bedtime.
She's also having fluoride treatment every 3 months. She does have a cavity and at some point this will need a cap or, if it deteriorates quickly, removing. So far it's stable and not causing pain so the dentist is just monitoring. It was detected when she was 3 last year.