IMO, the staring old man is a red herring. If you want to talk about eerie, how about that DeOrr was being served fries at an establishment owned by one of two RSO's in the area just two hours before he disappeared. Or, how about the guy who came along on their camping trip? Yikes.
I think the mom is grasping at straws because the PI is pressing her to remember anything, anyone, no matter how seemingly irrelevant, that could point at some kind of possibility of abduction and take the suspicion away from those at the campsite.
Anyway, was the old man even staring? If so, was it because DeOrr is a cute, beautiful boy who might bring joy to people? Or would it be because the boy was filthy?
(NOTE: Though DK says the store clerk who said the boy with the man in a black truck was filthy had the wrong time, DK admits the family went to the store, and PI says they stopped at the store after the cafe.)
Hopefully the old staring man remembers seeing the family, or someone at the cafe (even the RSO) remembers seeing the family, because, as suspicions grow, it would be beneficial to the parents that someone unrelated to their camping trip could vouch for when and if the toddler was seen in town that day.