Prosecutors have alleged that 45-year-old Christopher Alan Carlson of Indianapolis deprived his three grandsons of food and water and pushed, choked and repeatedly kicked them during hikes on Aug. 15 and Aug. 28.
The 12-year-old testified Thursday that his vision and hearing became altered and mountains appeared to sway during the 19-mile hike on Aug. 28, a day that saw temperatures as high as 108 degrees.
Investigators have said Carlson told them his grandsons were overweight and that he thought hiking the Grand Canyon would help get them into shape.
The other two were 9 and 8 years old at the time. They both testified on Wednesday and earlier on Thursday.