My new kitten acts strange at times. Are these early signs of a skittish cat?

My new kitten Alex is about 4 and a half to 5 months old. hides and freaks put everytime the door is open.

But then he comes out and is very friendly. Even with strangers once we get him out after the initial hide he is very friendly. We took him im my sisters bedroom and her friends baby met him and he was friendly. I can't figure out for the life of me though why he reacts this way to the door when someone comes in or we come up the stairs. HE was literally under my bed when I got home from work today and he slept with me these past 4 nights since he got home and usually loves me. But whenever that door opens he runs and hides or just bugs and with his eyes wide open sometimes. Long story short, is this an early sign of a kitten who will grow into an adult cat and hide all the time and be anti-social with people? I wanted a very friendly cat. He seemed that way at the foster house.