If you want to switch to a wallet that uses a seed. You can go with Electrum (can't run a full node though).
Can't he generate a new wallet in Electrum, save the mnemonic phrase, obtain the master private key from the wallet file and import it into Bitcoin Core?
I am not sure if Bitcoin Core allows only to import individual keys. Edit: It doesn't. Importing a few address would do the trick but might not be worth it.