Staining on teeth

I am in desperate help. My son is almost 3 and for almost a 2 years now this strange dark brown staining has been showing up on his teeth. He has been to the dentist 3 times. She doesnt know what it is , she might think its from eating too much green food like kale, but my son is such a picky eater he doesnt eat anything green. I have had a hunch it could be his vitamin that i switched him to. He will not take flinstone vitamins

So i had to find another to sneak into his juice at lunch. He takes enfamil poly-vi-sol multivitamin supplement. Is my hunch right?? Are there any other vitamins out there i can try??? Any advice for this worried mom???