Is it permissible for the family to boycott a brother who has boycotted his father, because the father believes this is necessary unless the son returns?

It is not permissible for children to abandon their parents. Filial piety is obligatory in all circumstances, as Allah Almighty says: "And your Lord has decreed that you worship none but Him. And to parents, show goodness." If the brother insists on abandoning and boycotting his father, then it is permissible to abandon and boycott him. This becomes even more emphasized if the father commands it. This is not considered a severing of kinship ties if the intention is to deter him from disobeying his parents, for disobeying parents is one of the greatest sins. The father is advised to improve his treatment of his son and guide him with wisdom.