and keep copy of a few addresses in case he wants to invest more?
Is this a safe way, because i don't want my grandpa to have bitcoin on his computer which is super slow (probably virus).
Yes, your procedure is right.
Just be sure to first install Electrum on a device which has no malware. It could be your own computer, a USB stick connected to a trusted device or even a linux distribution, in a USB.
Then, save your wallet, write your seeds on several piece of paper, along with addresses and private keys. You'll done!