Piranhas in general are very timid shy fish. It's a survival instinct thing. Give them some more time in captivity and they'll start to act more outgoing and unafraid of people.
Mike's right. Especially if he's wild caught, he'll be skittish for a while. Keep the lights dim and let him have places to hide when he feels spooked. He'll come around eventually, just be patient.
I have them for about 2 months, there are 11 of them red. I hope that this will no longer be fearful because of their almost not visible, when I walk through the aquarium, they always hid.