The person who "simply has their names on the deeds" of the land is "the owner" of the land.To have become the owner, someone must have transferred ownership to them, and from what I can gather, the parents have actually done that.So be clear, the son owns the land eventhough he probably doesnt own the house. The parents own the house but they have no planning permission for it (the son does) and the son may claim that he never gave them permission to build on his land or he may claim that the parents built the house as a gift to him on his land. The planning permission will bear testament to this.
The parents therefore built a house on their sons land (withouth planning permission) The son had planning permission, the parents did not have planning permission. The parents have created a huge mess for themselves. Sounds to me like they were attempting to manipulate the planning process and it has backfired royally
I guess the parents could take eviction action against the son but that will be very complicated given that the house is built on the sons land
and they will also have to admit that they had no planning permission to build it in the first place.. If they claim the house is theirs, they will have to admit that they have no planning permission and may be required to knock it down.The only option would be to admit that the house is their sons.