What rights does my have if he father built a house on and has occupied land owned by my grandfather for the last 20 years but a month ago the land was sold to another family member?
The new owner can force your father to move the house and leave the land if the new owner wants. Living on another's land, even building a house on it, with their permission does not give you any rights in and to the land itself: you are there solely with the owner's permisson. That permission was personal to the owner granting it; unlike an easement on a deed, it is not "part of" or "attached to" the land and does not bind later or subsequent owners. A subsequent owner can withdraw the permission. Your father should try to enter into a reasonably priced multi-year lease for the land under his house, to give him security; or see if he can buy that plot of land from the new owner.