Try mixing a bit of pellets into the seed, adding a bit more pellets and less seed each day until you are no longer adding seed. Pellets can be softened with little water or fruit juice. All birds have the potential to bite. An adopted bird can come to you sweet and loving or disguised as a little, green terror. Fear, misunderstanding, poor, diet stress, molting and hormones can affect a bird's behavior.
Hands frighten some birds.
As painful as it can be, try to ignore the bite. The bird is waiting for your reaction. In his other home, biting may have gotten the only reaction that he could get. Pushing into a bite, rather than pulling away, proves very effective. This is a little tricky to do the first few times, as it is not the natural response to a bite, but it does work.