I agree that they must be genuinely worried about his safety (plus, LE wouldn't confirm whether or not he had a gun). But they could also be worried about the fact they are in over their heads and have made numerous mistakes (even if not intentional) allowing all this to happen and waiting too long to speak. I don't think they are using this as a distraction, I think it got out of hand and now they realize they have to sort of 'come clean' about it as best they can because 1) he's not at home and the police will realize that eventually if he is charged 2) they cannot reach him and are worried about his safety 3) they could be implicated for lying about his whereabouts or actions if they don't speak up about it now.
I think they had trusted their son's story, didn't expect this to blow up which it only did last Saturday, then he said he said he was "going on a hike" on Tuesday to clear his head and they dropped him off or let him do that, and then that day he becomes a POI and it starts unraveling quickly, they can't find him, by Friday they realize they have to tell someone all of this. I'm not saying it's all been intentional, but it's all been really poorly played. I think now they're realizing they don't trust his story, and he is in real trouble, both legally and mentally and very well could commit suicide.