I'm really very sorry to hear about what's been happening with your brother. Of course, there's a slim outside chance that some of the funds might be recoverable, especially if the hacker passed them through OKX, as OKX usually investigates funds that are in one way or another connected to hacks or scams. They have frozen accounts in such cases, even during the FTX hack. You might want to contact OKX support without any further ado and provide all the evidence that you can gather, such as the transaction record or wallet address of this hacker. As for Bitcoin, it is hard to trace, especially if this Bitcoin hasn't moved to this date, but trying some blockchain explorers would do great. You can also go to the police and file a complaint, and get the services of a cybersecurity expert. Keep pushing, maybe then the exchange will cooperate.
Centralized exchanges can seize funds of hackers when they receive solid orders from governments or reports from customers. Sometimes they proactively do this in too big and famous hacks like Axie Infinity hack years ago.
As a user, you can send your support ticket to exchanges where you trace the hackers' wallets and stolen fund movements. To be serious and increase your chance of success in getting support from centralized exchanges, you must prove that you own these cryptocurrency with signed messages from private keys you own, and if possible you as a victim can file a report to police for their investigation on criminals.
Chance to succeed is not too high and the procedure surely is complicated.
Binance Recovers $5.8M Linked to Axie Infinity Hack