My toddler has a yellowish ring around his neck. he's had it since at least his first couple months after birth. it's very faint. what is it?

Yellow ring: You son may have increased levels of beta carotene in the blood and manifests itself as carotenemia of the skin. It is a benign condition and is caused by eating green and yellow vegetambles.

Yellow veggies...???: This is a difficult question to answer without seeing the boy and what the rest of his skin looks like especially the palms of his hands. Does he eat many yellow vegetables including carrots children may develop carotenemia which causes a generalized yellowish discoloration of the skin especially of the palms. It does not cause yellowish color of whites of the eyes which is danger sign.