How long would it take for us being married to fix his paper?

There are a number of concerns, since you met him when you were a minor and conceived a child with him before your 18th birthday. In addition, it is unclear whether he will have to consular process and seek a waiver, which can take a lot of time, create frustration, and may require him to live outside the U. S. for the next ten years. As a result, I strongly recommend an appointment or teleconference with a competent and experienced immigration attorney, before there are any more complications. Good luck.

Best wishes for your plan to become married. Although not clear from your email, I presume that you are a U.S. citizen and that your fianc? entered the U.S. lawfully and with inspection (regardless of whether he may have overstayed his initial visa). The USCIS is inconsistent in its application processing time, but for properly prepared and fully documented application packages, the USCIS's Atlanta Service Center now is adjudicating cases filed approximately eleven months ago. While that may be a reasonably good indication that a newly filed case will be adjudicated in about 11 months, the USCIS is inconsistent in processing times, with some cases concluded significantly earlier than eleven months. After filing of an application, a foreign national spouse would be lawfully present in the U.S. and in approximately 90 days could expect to receive an Employment Authorization Document (an "EAD" or "work permit") authorizing employment while waiting for the adjustment of status case to be concluded so that he will become a Permanent Resident (will get a "Green Card"). Of course, there are other details that determine eligibility. It would be wise for you and your fiance to consult with an immigration attorney who, after learning all of the relevant information, could advise about eligibilities, options and strategies to attain your goals, and who then could offer legal representation in the often complex application process.