Use querybuilder on the recordsource of the subform.
If you've added * fields from the table to your query, add the autonumber field to the query separately , then uncheck its Show property and set its Sort property to descending. (Both properties can be found in querybuilder). You are adding an extra field to the query which is unseen and is merely used to sort the results the way you want them.
If you've added each field individually to the form's query, then find the autonumber field and set its Sort property to descending. (In this scenario, you have the field already available so all you need to do is to have it sort the autonumber field descending.
HTH