Poor baby. If he is that small he was probably taken away from his mommy too soon. You should have him on kitten food not regular cat food. He is meowing cause he misses his mommy and siblings if there were any around.
He has to get use to his surroundings.
Like momofcats said, he is also scared. Just have patience and when he starts meowing, pick him up and tell him, "It's okay" and give him lots of love. In the end it will pay off.