Of course, he is your boyfriend's uncle. If you refuse him, it may affect the feelings between you and your boyfriend, so I don't think you should bypass your boyfriend and tell him about it.
First of all, you should tell your boyfriend about his uncle's behavior in detail, and ask your boyfriend what his uncle wants your ID card for, and whether he wants to do something legal and formal. If your boyfriend didn't know before, then you can ask your boyfriend to inquire or investigate. If your boyfriend knows, he is more obligated to tell you the reason for using the ID card.
Of course, in the end, I still advise you not to lend your ID card to your boyfriend's uncle. You can gently tell your boyfriend to be a lobbyist and ask his uncle not to borrow your ID card. I don't think there is anything you can't talk about. After all, you are just boyfriend and girlfriend. Even if one day you really step into the marriage hall, you should use your personal ID card carefully and not borrow it casually.
Secondly, if your boyfriend promised you, but it didn't work out, his uncle often came to borrow your ID card, but you don't want to lend it to him, then you can find some excuses to refuse him. For example, if you take your ID card to do other things, it is estimated that you will not be able to send it back in a short time, or your ID card is lost and being reissued, or your ID card is left in XXX, so it will be difficult to take it out.
Finally, I suggest that you must understand from the side, what is your boyfriend's uncle doing by borrowing your ID card? Can you see if what he did is legal? Do you need to take some risks? If there are some risk problems, we must consult professional legal experts in time to see if there are any measures to avoid risks, because some informal lending institutions can get a certain amount of money as long as they take out their ID cards. If his uncle runs away, will you pay back the money? Therefore, the issue of ID card lending must not be sloppy. We should face up to and understand the ins and outs of the use of ID cards, and don't let ourselves get into trouble and disputes.