Is it permissible to sell a plot of land to spend its price on the treatment of a son suffering from a chronic illness, or should it be left for him and his siblings, and is there any injustice to him or his siblings in that?

The father is responsible for his child's expenses, which include the costs of medical treatment, such as doctor's fees and medicine prices. This amount spent should not be burdensome, but rather should be reasonable and not cause him harm. Therefore, the father is not obligated to sell his land to treat his child, especially if the benefit of the treatment is minimal or uncertain. However, there is no harm if he chooses to do so. It is best to consult scholars and doctors, then perform Istikhara (prayer for guidance).