How to manage an employee when you don't have much experience in that particular field?

In my office, our manager is away most of the time, so some staff members have to report to me, including a guy in the IT department,I am good at my work but don’t have strong managment experience. I had a strong feeling that he was an idiot when he first joined us, everytime I ask him something, he just stares at me and freezes there, and I thought that it may be due to comunication problems, but now, other people in the office started to notice that he is uninteligent, I even got in an IT solution company to help him out and to train him and hand over the IT tasks to him and they got back to me and said that they don’t think that he is inteligent enough to take over, they even offered to explain the things to me so that I can take over, but I don’t want to have to keep on doing his work, I already have enough things to handle, and now its just me who cares if something that is IT related isn’t functioning, because he just starts doing somthing and just leaves it if he is faced with any problems, and doesn’t get back to me on it.

Can anyone please suggest to me on how to manage this type of person and what would be that best way to report to the managment about him, since at the end they are the ones that are paying him his salary, and its thier duty to get rid of him, if that is required.