my great grandfather never told anyone who his father was. She never married either. Can my great gr

In the Lords time and when more details are known or revealed - he will be sealed to both a Father and a Mother. The sealing ordinance only has significance and full meaning when child is sealed to both a Father and Mother. Every person will have that opportunity and blessing in the Lords time.

In some cases the Father or Mother to whom the child ends up being sealed - may not be the biological father/mother. 

NO, a child cannot be sealed to just one parent.