There isn't a foolproof answer to this question... Personally, i have never tried to convince anybody to invest in anything. Not my parents, not my brother, not my friends...
If it goes wrong, they will always feel like you were responsible for their loss, and that's a burden i don't want... But that's just my personal opinion.
Exactly and getting to convince people to invest in a very highly volatile asset like cryptocurrency is a very risky one because at some point, you might be blamed directly or indirectly if things don't go as planned.
If I'm not wrong, you said you already told him about bitcoin and it's difference from other altcoins as well as the risk involved and how bitcoin isn't a get rich quick scheme, then I will say that you've already done more than enough and there is nothing you're expected to do again, anything you do after now, might seem like persuasion and at some points, he might want some sort of guaranty which I'm sure you don't have to give except you're willing to repay him if he loses money.
Just let him make his decisions and possibly don't talk about it again to him except he ask and you can pick it up from them and if he finally gets interested, then you can teach him how to go about it and the safety tip with wallets and also possibly introduce him to the forum.
Just my opinion though.