I just did a google search for "August Reiger pictures". In all of them he is smiling. Now look at his last pic taken by his father on the trail, attached below. He looks so sad. It is very eerie.
REMEMBER, he was the one who wanted to go on this trip. This entire Ecuador trip was because HE wanted to do it. He wasn't dragged along - he initiated it.
My guess is, he's out there in the jungle.