Is this residential or commercial?
Are you the tenant or the landlord?
Prior to Oct 2015 there was no law on smoke detectors in single let dwellings but there was in lettings of multiple occupation.
Was is a hmo?
Has your Don made a claim against the landlord or demand for return of the deposit?
Without seeing the document I can only go by what you have commented previously. If it says the tenant is released from all liability, costs, claims, expenses, obligations whatsoever in relation to the incident and the tenancy agreement then the landlord should not be able to sue your son.
I hope this helps. Do let me know if you have any questions or queries. Please can I ask you to accept / rate my reply, many thanks.