Question about Setting up two Accounts

[Sylve]

You're best off setting him up on his own SE account.
Otherwise you'll get hit with account sharing lockouts, since technically the account belongs to you but your brother is playing it. (you'd both be logging in with the same SE credentials, which i dont think you can do simultaneously)

There are no issues whatsoever with using your Debit card to pay your sub and his sub on separate accounts, so go ahead and set up an SE account of his own. You can use your Debit card safely to pay his sub and then switch the payment plan to his card when he gets one.