From the Oregonian article:
http://www.oregonlive.com/portland/index.ssf/2010/06/kyron_hormans_blended_family_f.html
Kaine's adoptive dad, R. Neil Horman, says Kyron's disappearance has been a strain on the parents.
"The loss of a child like this is devastating," he said.
BUT...it also says earlier in same article:
More than a year after they were married, on Dec. 8, 2008, Kaine and Terri had a baby girl, Kiara.
Her name reflects a family tradition of Kaine's biological father, Kurtis Mengel, who has a twin sister, Kari Mengel Hoagland. Mengel and his wife, Kristi, named their sons Kaine and Kristian, now 32.
Just a guess on my part, but maybe his parents split up and his mom remarried a man who adopted Kaine? I know of plenty of people will remarry and have their new spouse adopt their child(ren). In fact, Terri's second husband adopted her son from her first marriage, even though her son is now living with his biological dad - not his adopted dad.