Have you ever take a course or read a book about Early Childhood Development? I highly recommend it if you haven't. Children's imagination and ability to distinguish between real and imaginary is not concrete yet. So having imaginary friends is the normal outcome of that developmental stage. Is he an only child? Sometimes more frequent with them too.
My oldest daughter, alone until the age of 10, had a long time imaginary friend. I would ask about her...how she was doing, where she was, what she was up to. I would even set the dinner table for her 'friend' and asked what she would like to eat - and yes, obliged and put food on her plate. I did not learn this from my family of origin. We did not need imaginary friends with 7 siblings running around to engage with. So I was shocked by my own daughter engaging with such 'nonsense' at the time. So was relieved to know it is considered a normal part of development.
My point is that I wonder what would happen if you accepted his friend and then you can talk to him about why his friend is being mean/rude/pushing, etc. It is certainly worth a try and it might just make him into a better friend to have around.