Is there a way I can help my boyfriend with his citizenship papers if I’m about to turn 18?

If you are a U.S. citizen, and if your fiance entered the U.S. lawfully and with inspection (such as with a student visa or other visa), then when you become married you and he will be able to pursue a marriage-based Adjustment of Status application so that he may become a Lawful Permanent Resident (get a "Green Card"). This is true regardless of whether he may have overstayed his initial visa or even worked without authorization. If he did not enter lawfully and with inspection, then multiple other issues would need to be addressed. As a Permanent Resident, he would be able to visit his parents abroad and then return to the U.S. After being a Permanent Resident married to a U.S. citizen for three years, he would become eligible to apply to become a naturalized citizen. It would be wise for you and your fiance to consult with an immigration attorney who, after learning all of the relevant information about both you and him, would be able to advise about immigration eligibilities, options and strategies, would be able to advise about costs and fees associated with the application process, and would be able to offer legal representation in the often complex application process.