The tenant seems to be referring to the standard period of 90 days under the Residential Tenancies Act. However the definition section says "residential premises means any premises used or intended for occupation by any person as a place of residence". I doubt he is living there, so the act does not apply and the notice period would be under the Property law Act, which applies to all leases other than residential tenancies. The act says "
215 Sublease for same or longer term as term of superior lease
(1)This section applies to a sublease under a lease (the superior lease) if the sublease is entered into, or purportedly entered into,—
(a)by a lessee under the superior lease; and
(b)for a term that will expire at the same time as, or later than, the time at which the term of the superior lease expires.
(2)A sublease to which this section applies does not, unless a contrary intention appears, operate as an assignment of the superior lease to the sublessee.
(3)If the term of a sublease to which this section applies will expire later than the expiry of the term of the superior lease,—(a)
the term of the sublease is reduced and will expire at the same time as the term of the superior lease; and
(b)the sublease cannot be registered under the Land Transfer Act 1952 but is to be treated for all purposes as creating an equitable estate in the land."