Sorry for the delay.
It seems that no notice to vacate has been served as yet. The landlord says that the house is likely to be put up for auction.
Thanks for your reply.
The Landlord is free to sell the property BUT any Buyer takes the property, subject to the Tenancy agreement in favour of your Son and Daughter in law. From your Son's point of view, nothing changes, other than they would pay their rent to the new Landlord.
The new Landlord can NOT terminate their Tenancy, just because he is the new owner. So, your Son is entitled to remain living in the property for whatever period is left on the Tenancy Agreement. It might well be that any new Landlord will be happy to enter into a further Tenancy Agreement with your Son once the existing one ends, but I can't of course say this for sure.
I hope this assists and answers your question.