Good Morning J., Are you sure its tartor on his teeth? If you have brushed this much and it's still there it probably isn't tartar. It could be juice stains of something likt that. Our eldest son took a liquid vitamin when he was little and it turned his top teeth a brownish color. Was yucky looking.
Has he fell and bumped his teeth lately? Our Gr son Zane 2, fell and really bummed up his month a few months ago. I called our dentist and he said to watch him and if his teeth started to turn gray to get him in, as he had damaged the roots. HE was fine but it took many days for them to tighten up so he could eat again.
I would call a ped Dentist and make an appointment. They are so good with little people will show them what they are going to do before they do it. Corbin did awesome on his first visit, the let him spin the cleaning pad, spray the water etc.. as did our other gr kids.
God Bless J.
K. Nana of 5
PS It tastes yucky bit I guess you could see if he will let you brush his teeth with a little baking soda and wet the brush with peroxide. Will bubble alot.
Teach him to Spit first though...lol No swallowing.