The Santa Maria man accused of killing his girlfriend's five year old son pleaded not guilty today. 24 year old Juan Francisco Nunez was arraigned Monday afternoon in a San Luis Obispo courtroom. County prosecutors filed felony involuntary manslaughter charges against him last week. They say Nunez buried five year old Jesse Alvarez in the sand during a July 4th celebration at Pismo State Beach. In addition to the manslaughter charges, he faces charges of felony child endangerment causing great bodily harm. Nunez is expected to appear in court again on August 11th.