My pup does this too, and unfortunately pups put everything in sight in their mouths. Careful though with stones because they could cause a blockage.
We have done a lot of work around the command leave it. Mind you we have yet to master it, so I'm also pulling stones and neighbourhood garbage out of his mouth on a regular basis. Just be really vigilant so that you can remove stones from his mouth before he has a chance to swallow them.
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