My husband has a 5 year old with son with a girl he had a thing with. She loved him and therefor she got pregnant and told him the child was his. She put him trough the ringer cause she wanted to be with and she realized he was never going to be with her she signed her rights to the child away. Her mother tried to get guardienship of him but because he had a father on the birth certificate, my husband was contacted and given custidy. Last year after speculation that the kid wasn't his we took a dna test and it was true. So he spoke to the grandmother and his not mine I think you should help out but that did not work out beacause the kid was not taken care of . Days of visit would result him losing weiht, he would come back with nasty matters, they would put things in his head which would problems so we stop it .this went on for 2years finally we cut her out of his life. Now she is using all the information he shared with her by suing him for custiding claming he is not his father and we have refused to let her see her grand son all these years.
Can he loose custidy of that bay since he is not the boy's biological father?

In almost all US states, he will have no legal claim what so ever and will always loose to a real bio parent. There are a couple states that if after a certain number of years, paternity can not be challenged if listed on birth cerficate. You need a lawyer fast. While not moral, if you did a DNA test no one should have ever been told if you did not want to lose the child