If you haven't already done so, I suggest that the Parish Council meets with a senior planning officer to discuss the status of the site and whether any future development would be acceptable. Whilst a planning officer can't give categoric promises for or against, as every application must be considered on its merits, they will usually voice an opinion on the matter and can explain the implications of the Local Plan for future development. In general terms, a former dwelling (assuming the pub was also a residential property) may be replaced with a new dwelling, which may be larger than the one it replaces, but I presume that the owner is seeking something more than a single dwelling.
I have similar sites in two of my parishes that have been untouched for more than 30 years, both former school buildings now in private ownership; one cleared, the other with the derelict buildings still standing, but gradually falling down. This sort of impasse can be very frustrating, especially on a prime site in the heart of a village. If the owners and the planners cannot agree on a solution, there is nothing the Parish Council can do.
If the site deteriorates to the point where it becomes an eyesore and/or health hazard, the principal authority may serve enforcement notices using their powers under Section 215 of the Town & Country Planning Act 1990. This will force the owner to improve the site or face penalties for not doing so. At the end of the process, the principal authority may improve the site at the owner's expense. Some councils are more inclined to use these powers than others.