Up till now it seems that you have been experiencing his emotions, and sharing his thoughts, without his being any the wiser.
Harry Potter and the Order of the Phoenix
The reason why I ask is that I saw another example from Collins dictionary, which uses 'you' instead of 'your'.
We could have stolen the original from the warehouse without you being any the wiser.
So, I'd like to know if both 'his' and 'him' can work in this phrase? And any reasons?
As @RubioRic noted in the comment, When to use an object pronoun or a possessive adjective before a gerund says:
The basic "rule" is that formal written English prefers the possessive in all circumstances, while informal, conversational speech prefers the objective.
Given that the stories are set in and around a school in Britain with a long history, one would expect the language to lean towards the formal. Thus here the possessive is used, rather than the objective.
Up till now it seems that you have been experiencing his emotions, and sharing his thoughts, without [his being any the wiser].
We could have stolen the original from the warehouse without [you being any the wiser].
Yes, they are both fine. The subject of gerund-participial clauses like these can be either accusative or genitive; it's a free choice, though the genitive is characteristic of formal style.
