He has about $10,000, and I think he wants to buy Bitcoin with it (that's a lot of money here in my country). The problem is that he doesn't know anything about Bitcoin, nor does he know anything about storing Bitcoin in a non-custodial wallet, or anything like that. So, I'm afraid that if I advise him to buy, he won't be able to store Bitcoin properly, such as by not keeping his seed in a secure place or giving the private key to some scammers, etc.
What do you think? Should I advise him to buy, and what are the basics I should teach him? Or is it better to let him learn on his own and make his own decisions?
If he is investing the entire $10,000 in bitcoin, I would tell you that it's too risky to let him learn anything on his own, I believe you are his brother for a reason, and it doesn't matter whether he is older or you are older, at this particular moment and Time and based on this subject, he is totally your responsibility as long as you have been in this space long enough and understand the dos and don't better than him who is just taking his first step into the water..
Teach him thoroughly on how to secure his bitcoin using non-custodial wallet, if possible, I would advice you advice him on the impotance of getting himself a hardware wallet, because if I have $10,000 to invest in crypto for the long term, I wouldnt be holding the crypto on any online wallet even if it its a non-custodial wallet, it is commonly said that one can never be too careful and it's a fact.