I don't think 30,000 is excessive for a 2 kanal property in Y block. The tax varies depending upon location of property, its size and estimated rental value.
You can request cantonment board for a re-assessment based on the fact that the property is in your grandfather's name and he is an unemployed senior citizen. But it is at the board's discretion how much exemption they grant.
As far as tax is concerned, I consulted my lawyer once regarding this and he said property tax is a provincial tax and it is different from income tax which is collected by federal government. The two are unrelated.