Baby teeth struggle, possible Carriers? TMI

What could be the problem with his teeth and what can i do?

Took him to the pediatric dentist 2 months ago for his first checkup and a possible lip tie, dentist just told me at his age they do nothing.

Not even a week later i noticed stains on his teeth so i took him to another dentist because i didnt feel comfortable with how the first one missed the stains. This new dentist told me it is fluorosis and it could be due to the vitamin D tablets that i gave him in his first year of life. His toothpaste is flouride free and i only introduced him to salt 2 months ago. Dentist said they can do nothing about it and the milk teeth will fall out eventuallyso nothing to worry about.

Still it kept bothering me that the stains are there and i googled. They said backing powder gets rid of stains. I did try to get rid of the stains with backing powder twice. I noticed pieces getting off but they were resilient. He was fighting me so i stopped bothering him. Just continued brushing his teeth twice and after any meal for at least 10 minutes. This is me frustrated about the stains. Luckily he has always enjoyed brushing so it doesnt bother him that i do it for long or give him a toothbrush to imitate me.

Anyway. Today he passed out so i got in there and wanted to scratch away the stains in his sleep with a dental tool. I noticed they were getting off like whitish stuff but also noticed the stains were not on top of the tooth but rather the place where the stains were, was a depression in the tooth. Which meant he had no enamel in that part so they were not stains but rather his enamel was eaten away closer to his gum. The closest part to the gum has the most white parts. I immediately stopped the procedure.

Now am worried either the supplements i gave him ate away his teeth, or he has carries. Could his lip tie have contributed this?

What do i do? Am at loss because the dentists dont even touch him because he is young. He doesnt cooperate either (doesnt cry but wont open mouth) so usually i am just describing what i noticed and am no professional and i try to open his mouth so they see which usually is a second and i am never sure if they have seen well.

Am frustrated honestly. I have never had carries all my life so i dont even know what to do or if he can be helped. I was determined to have a toddler with nice teeth but now i feel like a failure. Like how could this happen with all the brushing and dentist appointments i make.