Since then, he has revealed to my mother that he indeed does have jinn under his control. However, despite knowing this, my mom and others in our family still ask him to "sense" things such as who is doing magic on someone else in and what the person who is doing magic looks like. Obviously, now it's confirmed it's not through Allāh (SWT) on which he says such things. My only concern and question now is that is it Shirk if one asks him such as my aunts and mother does on their part since it's like thinking he has knowledge of the unseen?

You should encourage your mother to not consult him and ask him about such matters. Remind her that she should be a servant of Allah and only go through the channels the Ahlulbayt (a) have recommended. So do your best to convince her.