Can a man bastardize a child that was born during a marriage that is not biologically his after 12 years?

Can my husband file a paternity suit against me to prove my 12 year old son is not his biologically to avoid paying child support even if he knew he was not the father and signed the birth certificate with the agreement that the biological father was never to know? My son is 12 and was conceived during our first year of marriage while my husband was in prison. I told him I had a one night stand and ended up pregnant and offered to file for divorce while he was still incarcerated. He refused saying he would raise the baby if I never told the biological father. He signed the birth certificate and we remained married for 12 years raising him together along with four other children. Two being ours born after this child. Now he is trying to have paternity test done to prove the 12 year old isn't his to reduce his support order. Can that happen? Also, I have the letters from while he was incarcerated discussing the child and the agreement made.