Dirty Little Neck

When my sister and I were little, and our own kids were little, my mom called it "brown sugar neck".
There's nothing sweet about a dirty little neck, but kids are far more happy to let you wash their necks when you're getting the "brown sugar" off than telling them to scrub off the dirt.
Soap, water and a washrag.
This might sound weird, but you might try some cold cream. It melts away mascara and makeup. You can just put some on and leave it for a few minutes and tissue it off before bathtime.
Little kids can get grimey necks, especially in the summertime, so just be sure to help wash it every day.

Actually sugar is something that is used to scrub areas of skin. I like the idea of calling it a "brown sugar neck" and just ask your child to use a "sugar scrub" to clean it (sugar in a liquid soap).

let him go swimming.. the pool water will wash it away.. then show him how to wipe under his chin everyday.. good luck

Astringent for sensitive skin will do the trick.

For how long have you scrubbed? With my boys' knees, if I don't scrub them daily, they get pretty dark and it takes a while (days or weeks) to get them back to normal. Could it be tanned from sun exposure?