LYNN – The Melrose man accused of driving under the influence of drugs and killing a Lynn taxi driver on Friday night is expected to be arraigned today at Peabody District Court.William Halpin III, 25, has been charged with motor vehicle homicide while under the influence, operating a motor vehicle under the influence of drugs, second offense, possession of a class D substance with intent to distribute, speeding and marked lanes violation.If convicted, he could face up to 15 years in prison.He allegedly slammed into two vehicles on Route 1 north in Lynnfield in front of the Fat Cactus restaurant at about 8 p.m., killing Victor Vargas, 42, and seriously injuring a 6-year-old boy.Vargas, a native of the Dominican Republic, had been standing in the breakdown lane with Patricia Ediamen, the driver of a Chevy Cobalt that he allegedly rear-ended, when Halpin?s 2004 Chrysler Pacifica SUV allegedly hit the Cobalt and then Vargas.The 6-year-old boy inside the Cobalt suffered a seriously injury to his arm and was taken to Union Hospital in Lynn and later transferred to Children?s Hospital in Boston. His condition was not released on Sunday night.Two other passengers inside the Cobalt, including a 3-year-old, were not injured. State Troopers say they found several bags of marijuana that Halpin had allegedly tossed from the SUV after the crash.Vargas had been working for Garcia?s Taxi in Lynn for four months. He leaves behind an 18-year-old daughter, an 8-year-old son, his wife and parents.Calls placed to Halpin?s home on Sunday were not answered and Vargas? family was not available for comment.