Not a good idea to self medicate, Antibiotic resistance is now a major issue and taking them without input from a medical professional will only serve to make antibiotics less effective or even useless going forward. Just thing what it would be like if there was no treatment for Syphilis, some guys on this site would no doubt get boned up at the thought of dying from such an painful infection. But I think most guys would agree it's critical that we have antibiotics that continue to work and save lives.
I appreciate your issue with the BF, when I was in my last relationship I used to worry about this alot, thankfully towards the end we were both cheating so when he got STIs he just assumed it was from his own activity! If it comes to it, confess a drunken blow job!
Enjoy