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Hi, Welcome to Just Answer. I'm Chris, a small animal Vet based in the UK. I am reviewing your question and will get back to you as soon as possible
It is my understanding that normal brown walnuts pose very little risk, black walnuts (a different variety) are more dangerous.
Either way such a tiny piece is unlikely to cause any harm so I would try not to worry, he should be absolutely fine.
I hope that puts your mind at rest?