So he's been in the yard with the rocks and then 4 days ago it suddenly occurred to him that the rocks might be turtles? Maybe he saw a bug or a toad run under the rocks?
Any chance he might just lose interest in the rocks if given enough time? You know, like a favorite toy that no one else is allowed to play with, after a while it gets discarded if no one else is interested... that's part of the fun.
I mean since they are just rocks and not turtles or something and will respond to his attacks... maybe he will get bored with them.
Sorry I don't have any advice on how to overcome this... they can really obsess over some strange things... ( sorry, but the mental image makes me chuckle
. I can just picture him attacking these rocks with everything in him...)