There's a few things going on on this thread and a lot of misinformation
Its not blaming the mum to say an illness in pregnancy is a potential cause. It's not your fault, it's just one of those things, and an illness is a known cause. Having said that it could be anything as I said above so dont fixate on that as the cause. It's normally something happens at the point of calcification or enamel development, so a knock, illness that type of thing. However it can just happen, and like I said pointless finding the cause as it doesn't change the hypoplasia.
Antibiotics are unlikely to be the the cause, tetracycline cause a specific type of staining but not antibiotics in general.
There are 2 conditions : hypoplaaia and fluorosis. These are different things and I can't say which one your DS has without seeing his teeth. One is weaker enamel, the other is cosmetic
Adult teeth are yellow, much more yellow than baby teeth. Again this is an entirely different thing, neither hypoplasia or fluorosis just normal yellow teeth. Can be sorted if wanted by whitening in the future.
A dentist will be able to tell just by looking at the teeth whether they are hypoplastic or not. If a consultant has said that certain teeth are unaffected certain teeth are unaffected
Either way lots of children have mottling of their teeth. It can be sorted but when they are older.