"For some reason can't transfer it directly to his account overseas (something to do with security codes, authorized payees and expired cards)."
Don't become someone's financial intermediary. Find out exactly why he can't transfer the money himself, and then if you want to help him, solve that problem for him. Helping him fix his issue with his expired card, or whatever the real problem is, would be a good thing to do. Allowing him to involve you in the transaction, would be a bad thing to do.
Possible problems which might be caused by becoming directly involved in the transaction:
-The relative is being scammed themselves, and doesn't realize it / doesn't realize the risks, and either wants you to take the risk, or simply thinks there is no risk but needs administrative help.
-The person contacting you is not the relative - perhaps they are faking that person's identity, and are using your trust to defraud you.
-The person is committing some form of fraud, money laundering, or worse, and is directly trying to defraud you in order to keep their hands clean.
-The transaction may be perfectly legal, but is considered taxable in one or more countries. By getting involved, you might face tax filing obligations, or even tax payment obligations.
-The transaction may be perfectly legal and legitimate, but might accidentally get picked up as potential fraud by a financial monitoring system, causing the funds to be held, and your account to be flagged for further investigation, creating headaches for you until it becomes resolved.
There are possibly other ways that this can go awry, but these are the biggest possibilities I can think of. The only possible 'good' outcome here is that everything goes smoothly, and it works exactly as well as if your relative's "administrative problems" were solved first, and the money went through his own account.
Handwaving about why your account is needed and his is faulty is a big red flag. If it is truly just an administrative issue on his end, help him fix that issue instead.