IR did suggest in his Nate interview that he was surprised the parents were named as suspects and had assumed DeOrr got lost or taken by an animal. This makes me think (if he was being truthful to Nate) that any information he has pertaining to foul play is incidental / circumstantial, rather than glaringly obvious.
We also heard he wasn't clock watching that day and so could not reliably confirm timings. Perhaps his "help" is limited to discussing the parents' behaviour, whether they searched much, whether he could hear them calling DeOrr's name. It should have been absolutely frantic: people running about, shouting, screaming, yelling, crying, trying to make calls, running, turning the campsite and vehicles upside down, desperately questioning Grandpa, searching the creek, down to the reservoir, back up again, chaos everywhere.
If it wasn't pretty much exactly like that then those parents are being, ahem, WAY less than truthful.