There is no alternative but to act. In our everyday lives we often think or act as if morality were an institution that we more or less agreed to, while every so often allowing ourselves to take a rest from the burden of moral obligation. However, if consideration of one's son's need really is the decisive actor, then what one is in effect saying is that keeping the note is, after all, the right thing to do in this case. What is implied here is a choice between moral considerations and non-moral considerations. But there can be no such choice.
Moral considerations override all other considerations.
Failure to act is proof of a lack of knowledge. The paradox of weakness of the will presents a serious obstacle to an account of moral knowledge. On the one hand, if we allow the necessary connection between knowledge and action to be severed, then we are no longer able to account for the motivating force of moral considerations. The only alternative, therefore, is to argue that the experience, however vivid, of seeming to see what is to be done is not equivalent to really seeing, fully appreciating the significance of the facts that are there to be seen. At least, that is what one has to say.