You can't.
AFAIK, Copay doesn't support importing addresses(WIF format) into the client. Copay needs the user to send the funds into a new Copay wallet. This is far better since you don't have to remember the private keys and all your addresses generated in Copay will be covered by the seed.
He could always recover his keys if he has a backup for his Bitcoin Core wallet.
But like ranochigo said, it's a bad idea since the keys derived from your seed and the keys generated from your Core wallets won't be the same.
You should send the funds you plan to use on mobile on its wallet.
Keep the rest in your core wallet. As opposed to try have the same private keys in both wallets, which is not really a good practice.