My husband is now a US citizens, is his minor child in Brazil a citizen? How do we bring him here?

No. He is not a U.S. citizen just because his father is a U.S. citizen. If the mother has legal custody over the child in Brazil then the father must go to family court in Brazil and try to gain legal custody of the child, which is probably highly unlikely unless he has evidence that he can present to the court which proves that the child would be better under his custody. (He must gain full custody by the way), and even then he would need the legal mother's permission to allow the child to move to the United States. It's an arduous, difficult process that will involve your husband going to Brazil for a legal process that can take up to a year possibly.