A 31-year-old San Luis Obispo man has pleaded guilty to causing a crash in Monterey County earlier this month that killed a 4-year-old boy and seriously injured his mother and sister, according to a statement released Tuesday by District Attorney Dean Flippo.Dion Gussner pleaded guilty at his arraignment to gross vehicular manslaughter while intoxicated, charges filed after he drove his truck into the back Dr. Christa Balch’s Honda at the intersection of Las Palmas Road and River Road on Aug. 9.Dr. Balch and her 2-year-old daughter Ava were seriously injured in the collision. Her son Sebastian was killed instantly.Gussner, whose blood alcohol level registered twice the legal limit when tested at the scene of the crash, claimed he had been sending a text message and never saw the Balch’s vehicle before he hit it. Gussner agreed to accept the maximum sentence allowed under California law for the charges, which will amount to 16 years in state prison, according to the district attorney’s office. He is scheduled to receive that sentence on Sept. 30.