About ten odd years ago Ken had an affair with a hairdresser called Denise and they had a child together. The actress decided to move on so they wrote out the child as well - the character found a new man & moved to Scotland.
Deidre knows about him, Deidre & Ken weren't together at the time.
I haven't actually seen the episodes where he goes to visit them unfortunately, this is a link to an interview with the actress who plays Denise http://www.itv.com/page.asp?partid=3958