The person selling the land has to show his right in the said land. If the agricultural land was purchased by any of his forefathers from his own earnings and the ownership did not transfer to him through will or otherwise, he cannot sell it except with the permission of all the other legal heirs. His son can challenge the sale to cull out his own share in the land.

on what authority has he sold the land

If the father has sold the lands for your family needs then sale is good on you. Or else you can claim you share of property in lands. Contact a good lawyer of your locality.

if agricultural land belonged to his forefathers then father would need consent of other legal heirs for sale of land . the son can file suit challenging sale of land by his father