I saw old Jolyon's buying his son's paintings not so much as a way of helping him financially but more as a step toward reaching out to him emotionally and regaining contact with his son. He keeps the paintings hidden in the same way he keeps his emotions hidden, and he doesn't want the family to know what he is about to do, because it is contrary to everything they stand for (that is, material security paid for by emotional repression). It was a way of having something of his son who has chosen happiness over material prosperity, a direct repudiation of the Forsyte way, and old Jolyon, in desiring to reconcile with his son, is about to do the same thing. Thus, JG has in the very next paragraph after the painting buying, (speaking of old Jolyon) "terrible yearning came on him to see his son." Then he goes to his son's club and contact after fifteen years is renewed.