I say, and with God's success: The sacrifice is obligatory for one who possesses the equivalent of 100 grams of gold, which is the threshold for zakat. If the mother or the son does not possess this amount, it is not obligatory for them. If one of them possesses it, it is obligatory for him. If both possess it, each of them must perform a separate sacrifice. If neither possesses it, there cannot be a sacrifice for both, but only for one of them due to the prohibition of sharing. And God knows best.