How do I get my kitten used to being outside?

I have a three month old kitten, since i got him he has been indoors. I want to eventually be able to take him out for walks on a harness. Ive tried to take him outside once at night, when i even took him out of the apartment into the hallway his body started to shake alot and he seemed scared and started looking up at me and meowing. then when i actually took him outside the building he got really freaked by the very few cars that were going by at 2:00am.and climbed onto my back. Im wondering how i can get him used to being outside, or if its too late to get him to even go outside and now hes just going to be an indoor cat forever.