When I take him outside, he always opens his mouth because he is not a fan of outside time. With this in mind, I took him outside today and tried to brush his teeth a bit with a soft toothbrush. I don't think the stains went away.
On a side note, his behavior is fine.
He eats well, he basks, he has his rep-cal calcium and multivitamin.
He eats dubia roaches mostly with a hornworm every once in awhile. He really loves colorful fruits such as strawberries which are also provided occasionally.
I don't think there is anything to worry about, but I just want to double check.