I believe it is Walter Hornbeck and was on the shawnhornbeck.com site. I will try to get the link.
From the google cached version of shawnhornbeck.com
http://72.14.253.104/search?q=cache:CK5mhIKNi9MJ:www.shawnhornbeck.com/+Shawn+Hornbeck&hl=en&gl=us&ct=clnk&cd=1&ie=UTF-8
FAQ's - Frequently Asked Questions -
Q: Who is Shawns biological father?
A: Shawn's biological father is Walter Hornbeck.
Q: Could Shawn have been taken by his biological father?
A: Shawn's biological father COULD NOT have anything to do with Shawns disappearance because he has been dead for almost 3 years.
Q: Is Craig Shawns biological father?
A: Craig is Shawn's step-father. Craig has known Shawn since he was about 1 year old. Craig is the only father that Shawn has ever known.