I'm training to be a T and the way I look at it is this: disclosure should be in the best interests of the client. So in this case, if it is likely he would be affected in session by his feelings about his mother, it would be important to be congruent with you so you don't mistakingly believe you have done something to upset him. If he had been unable to be present for you in session the ethical thing to do would be rearrange the session but it seems like in this case he did the right thing.