Sounds complex. But the will is clear so the land will need to be divided in two, one part to the son and one part to the nephew. I assume the nephew will need access road or drive to the parlour and a bit of space around it?
If son tells nephew he is selling the nephew has two choices, sell the whole farm in one lot and take a % of what it makes as value for the parlour. Or take ownership of parlour so executor transfers ownership to him. I assume son will need to fence off area owned by nephew so it is very clear what is not been sold and then he sells the farm less the parlour.
If the parlour is landlocked by the farm then the son will just have to sit down with nephews and solicitors and work out the fairest way to grant him an access route to the parlour. I think it would be easiest for a sale to give him a lane way and fence it off, despite some loss of land otherwise the remainder of the farm might sell for less than it is worth if the buyers are not clear on what they are buying and what rights stranger to parts of the farm.