Selling assignment land (govt. quarter) without son's consent

It is your self-acquired property and can be dealt in any manner as you like.  If you want authentic confirmation, File RTI to the authority that issued the Patta and request information under RTI as follows;

1. Please provide me a certified copy of laid down rules and regulations to dispose of the property for the necessities of the purchaser of Government land through  Assignment land patta.

Admittedly, the property assigned is not saleable, but only heritable. If so, how could there be the question of selling the property? You son's permssion is a secondary aspect.

Actually the government quarters was by bought by me in the year 1993. Transaction has been done through Bond Paper, hand written and signed by buyer and seller.

In 2015, when applied for Land Regulation scheme, Telangana government issued Assignment Land Patta for Land under Possession.

This is the complete picture and now I wanted sell. But, my sons are not allowing me to do that.

If, as stated,  you have been conferred with the right to transfer, your sons have standing to object to the sale.

Can you purchase "government quarter"?

If it is not saleable how in the first place did you buy it...

Prima facie it is a time-pass hypothetical and fabricated story.