Not sure a gang would take him. He can’t put any work in. He’s too small. He’s going to be a victim his entire time in.
And then there’s sex of course. :-)
He can always join a religious group like the Nation of Islam, which is active in prisons. Inmates either find religion behind bars, or join a gang for protection. Just because he's small, doesn't mean that they wouldn't take him in, just so they can extort him.