Considering the timing of his arrest to the announcement her body was found, I have a hunch he confessed. JMO
In the video (bottom one) at around 13:15 the reporter said investigators were nonchalantly talking to the boy and then placed him in handcuffs. He very well could have confessed or said something to investigators that if he was innocent, he wouldn't have known the details. MOO.
http://kron4.com/2015/07/27/santa-cruz-police-searching-for-missing-8-year-old/