It is all dependent on timing. If the mother became a US citizen before the son married, then the original I-130 will remain valid and will move (keeping the original priority date) from F-2B-->F1(when mothers becomes a citizen)-->F3(when son marries). However if the son married before the mother became a US citizen, then the original I-130 is null and void and the mother will need to submit a new I-130 for her son.