Is it a new Ken? He may have glue hair syndrome. Most of Mattel's current Barbies (and Monsters), especially those from Indonesia, suffer from it. The problem is that the glue they filled his head with after rooting it is unstable and has started to leak out.
Does it feel really oily or greasy, especially at the back of head? The hair clumping together at the roots? If you've already used dish soap, you've probably already got any hair gel out and what your feeling now is glue from inside his head.
Some things that
might get it out: Goo Gone, Goo Gone 2, L.A.'s Totally Awesome Orange cleaner. That's all I can remember right now. You might have luck looking for solutions for Monster High Glue Head. But this only solves the problem for a little while. The glue will keep leaking and you'll have to clean him again in a couple of weeks to a couple of months. The only way to truly get rid of the problem is to cut off all his hair, get rid of the glue from inside his head, and re-root him.
Hope this helps!