You can show him any similar fatal example from the past, where people did these kinds of stupid things and lost their money. He will probably be convinced in this way. Otherwise, keep Satoshi's statement in mind: “If you don't believe me or don't get it, I don't have time to try to convince you, sorry." And save your energy for people who are open to learning and correcting their mistakes.
I've shown him and given real examples of how people lost 30+ BTC just by keeping their seed on Email draft!
In his heart he knows the risk and knows he might be doomed someday (he said this to me while we were chatting!)
Also, he tells this to everyone and doesn't keep it as a secret (he has balls of steel lol)
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