Discolouration on sons teeth

My son is 19 months old and this past week I’ve noticed some slight discolouration on his top two teeth.

He only drinks water and on the odd occasion he has juice , he isn’t overly keen on sweet stuff but will have the odd treat every now and then when he feels , he absolutely loves having his teeth done and always has done , he will physically sit down with his toothbrush waiting for his teeth to be brushed and I brush them the recommended times a day for the recommended duration so I’m not sure as to why this has started happening.

The only things I can think is , we’ve not long moved and I must admit the water is different to where we lived before and he’s gone off milk all together even though I offer it him he will have a couple of sips and then disregard it.

Has anyone else had this even though you’re doing everything supposed to?

I have an older child as well and did all the same things and never had this problem and she absolutely hated having her teeth brushed so cleaning her teeth was more of a challenge than his and her teeth are beautiful.