If my former boss's credit card is overdue, he sends me a text message asking me to tell him, what should I do? Am I his sponsor?

See if you have issued a formal guarantee contract. If not, then he filled in your phone number when filling in the credit card information contact information. If you have issued a formal guarantee agreement, then you must accept this proposal and negotiate with the bank to bear the percentage of his overdue amount. If you don't know or remember, you can log on to the website of the central bank and check your credit report online to see if there is any relevant information in the guarantee column.