Search continues for missing autistic boy Robert Wood Jr. in Va.

(CBS/WTVR) HANOVER, Va. -- The search for an autistic 9-year-old boy continued through the night and began again Thursday morning in the woods of Doswell, Virginia.

Robert Wood Jr., who does not speak due to autism, disappeared while on a family walk in North Anna Battlefield Park, reports CBS affiliate WTVR. Captain Mike Trice with the County Sheriff's Office said that the boy was last seen about 150 yards from the North Anna River.

Hanover County Sheriff David Hines said 889 volunteers were trained and sent out to search a 2,000 acre area. Over 300 professionals were also dispatched to assist them. The number of volunteers over two days reached 1,600 people says the Richmond Times Dispatch.

Law enforcement officials were asking for more volunteers today, especially those who have already been trained, to meet at the theme park King's Dominion parking lot at 9 a.m. to continue the search.

Tracking dogs picked up the boy's scent near the river. Dive teams and kayakers have searched more than a mile upriver for Wood, and the river's water level had been lowered to assist the search.

Meanwhile, search crews used the canines, as well as nighttime thermal imaging, in an attempt to locate th child. Authorities said there was a better chance of finding him at night when the conditions were better for the search dogs.

When asked the odds of the boy being alive, Trice said this point was all that mattered: "We are going to continue this until we find him. ... We're operating under the assumption that Robert needs our help."

Robert has blond hair and blue eyes and was last seen wearing a red, long-sleeved T-shift, blue athletic pants and blue shoes.