All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
In Islam, repentance wipes out previous sins. Therefore, these two Muslim persons are obliged to repent for committing this great major sin, but there is no expiation for it. They can marry after truly repenting from Zina (fornication); for more benefit, please refer to Fatwa 89823.
The child should be traced back to the mother, and in relation to the man, the child takes the ruling of a step-child.
Fatwa 84788, gives the details of the rulings of such a child so please refer to it.
Allaah Knows best.