Welcome to HCM.
Looks like your grandfather is suffering from post operative Endophthalmitis
,where in there is an infection of the inner layers the eye(both anteriorly and posteriorly) .
There is a chance of infection spreading to all the layers of the eye. So the early surgical management the better.
I would also suggest to go for a second opinion
and if the infection is not coming down with both oral and topical and intraocular antibiotics then considering his age I would still recommend to remove the eye to get a complete relief of pain as using eye drops or taking oral tablets will not help him for a long time.
Hope this helps. Take care.