Considering the adults are discussing a missing and possibly murdered child I would say yes, something heavy is being discussed. But to be clear, you would find it LESS suspicious if LE were having this heated discussion with the kid? Is that not backwards?
In any case, there is nothing unusual or suspicious about someone wanting to be alone for a while in their grief -- particularly a kid, and even more so if the adults are talking about things he prefers not to think about. Before accusing someone of behaving unusually, it is best to consider what normal behavior would be. Sitting by yourself is evidence of one thing only: that at the moment in time that this image was captured this kid either didn't feel like hanging with the adults or was asked by the adults to go away. That's it.
In this particular case there is absolutely NOTHING to suggest that this kid was involved at all. Nothing to support this theory, including statistics or the few facts we know about this case. The only reason he is being mentioned here at all is that...
1. He is a teen, teens are scary
2. He is a male, men are both are scary and guilty
3. He isn't mom, and it couldn't have been her