What is the country of origin of your son? I am asking because the F2B category might be faster than F1 depending on the country. Anyway, you should notify the NVC that you are now U.S. citizen. He will convert to F1. However, he can opt out to F1 if F2B is more convenient. Please check the latest visa bulletin. Lastly, filing another petition won’t help at all. You don’t need to do it. A new petition will start from scratch and it will take almost 9 years.