Yes - ask HIM why he wants to wear them. Don't shame him. Be patient and wait until you get an answer. Then listen. Do NOT buy them. He is a big boy and he does not need them. Draw the line. He will be fine. Basically understand, empathize and then the answer is still no. It is not good for him to do that. It's not what people do. People don't **** their pants once they know how to use a toilet. Way it is. And diapers are an expense. The answer is NO. But you can still be nice about it and try to understand him.
Me and my wife decided to just continue providing diapers for our 2 boys even after potty training. We had them resumed bedtime diaper use until age 7. By that time both our boys knew how to change their own wet diapers as needed. So, if they feel like being in a diaper after school or on weekends around they house they do so. They both get excellent grades and are very well behaved.
No you would be enabling him in his bahavior if you go and buy them in fact it would be best not to talk about it at all, tell him he is a big boy and diapers are for Babies, has anyone else ie the Baby sitter allowed him to wear them? he has had diapers from her. Have a talk with the Baby sitter and say the same thing ,no diapers and no discussion of them, they are for Babies not big boys.Do not feed into it distract him if he is looking for them and do not talk about them at all.ask your aunt also not to let him have diapers has she a Baby in the house,?Good luck if he gets no attention this will go away.