Gray color along gum line

My son is 15 months old and we’ve been brushing his teeth 1-2 times a day minimum since he got his first two teeth. We use fluoride-free toothpaste. His teeth have always been fine, but in this last month right near his gum line is gray in color. On every tooth. And he has two teeth on the bottom that have a more prominent gray color stain. I’m super selfconcious about this for him, and just feel like a bad parent. We have multiple nieces and nephews and as I’ve talked to their parents, they’ve all informed me that they didn’t start brushing until 2 years old.. but my son eats so much food, drinks whole milk now, and has 12 teeth.. it feels wrong to NOT brush them?? So.. I’m just SO confused why his cousins teeth never looked like this without brushing, but his look like this with lots of brushing.. I’ve gone down the rabbit hole of google searches and can’t fathom that it could be tooth decay or anything. That would be so upsetting. And I’m not sure if we should try a fluoride children’s toothpaste, and maybe that would stop the staining? Anyone else see this with their kids? The only pictures I’ve seen of children’s teeth are brown / white stains and that is not the issue here. We have an appointment with the pediatrician in 2 weeks, so I’m hoping to get some answers. But just would love peace of mind earlier than that, if anyone could help! Can’t get a good picture because he’s too squirmy..