Is it permissible for the questioner to sell a plot of land he owns to improve his family's financial situation and buy a better house, knowing that his father is not pleased with its sale? And is selling it considered an act of disobedience to the father?

You have done well in your eagerness to honor your father, for in the father's pleasure is Allah's pleasure. Your father's statement that the land is yours and you are free to dispose of it is an explicit permission to sell it or keep it, and there is no disobedience (ʿuqūq) in either of these actions. Consult those with expertise about what is best for you, and if you sell it, use its price for a house you live in or another piece of land, for the Prophet, peace and blessings be upon him, said: "Whoever sells a house or property and does not put its price into something similar, it is likely that it will not be blessed for him."