Disregarding all emotional attachments and just looking at the value of the two lives at risk here, I think you have to save the son.
Let's assume that the main purpose of human life (and all life in general) is continuation of the species. If that is the case, then the son's life is more valuable because he has not yet served his purpose in life, whereas the mother- having given birth and raised a child to near adulthood- has served her purpose. By saving the mother and letting the son die you not only remove his potential contribution to the species but also nullify the mother's contribution.
I know that's a very cold and disconnected way to look at it, but in this situation two lives are in immediate danger and you have to choose quickly. Emotions will inhibit your decision making, so it's best to become objective if possible and look at the situation from a neutral point of view.