This is a very interesting question, because before Kumalo finds out for certain that his son killed Jarvis, he definitely has a foreboding that it was his son. Consider Chapter 13, which follows a mad chase around his son's former haunts to try and find out information about his son. The people they see tell Kumalo and Msimangu that the police came and asked about Absalom, thus hinting at his involvement in the crime. Note how Kumalo pays the taxi "with shaking hands" following these visits, clearly hinting at the unrest he feels inside, and the suspicion that his son is involved in the crime.
Note too how in Chapter 13, the trip to Ezenzeleni, the blind colony, Kumalo goes through a time of deep depression when he is not able to cling on to hope - his son is likely to have committed the murder and he feels that the tribe is broken:
He bowed his head. It was as though a man borne upward into the air felt suddenly that the wings of miracle dropped away from him, so that he looked down upon the earth, sick with fear and apprehension.
These events clearly prepare the way for the final confirmation in Chapter 14 that it was Absalom that killed Jarvis. Thus Kumalo is able to act stoically, because in a sense, he has already reacted to the truth that he, at least partly, knew was going to be revealed.