Thanks for your question. Welcome to the forum.
Syphilis is very, very unlikely. Syphilis is a rare problem in the US as present, including most commercial sex workers and other heterosexuals with multiple partners. The syphilis epidemic is concentrated primarily in men who have sex with men. So it is very unlikely your partner was exposed to syphilis. Second, if your partner had acquired syphilis, he probably would have had a penile sore (chancre). Third, as you were told on the community forum, a bump like you describe is not consistent with syphilis. As you suggest yourself, a sebaceous cyst or some other non-STD explanation is much more likely. Finally, even if were a typical chancre, it couldn't start to improve this soon. Without treatment, chancres last for at least 2-3 weeks.
Since you are concerned, I would suggest syphilis testing. Plenty of time has passed for your boyfriend to have a reliable blood test: with exposure in April or May, a negative test now would prove he wasn't infected and therefore could not have infected you. If for some reason he can't or won't be tested, you could have a blood test for syphilis yourself once 6 weeks have passed since you resumed sex with him. (Don't worry about an immediate examination. There is no "swab test" for syphilis, so it doesn't matter if your genital bump has cleared up.)
Whatever you do, stay mellow in the meantime. This is a very low risk situation with regard to syphilis, and neither your nor your partner's symptoms make me at all suspicious of it.
I hope this has helped. Best wishes-- HHH, MD
Thank you so much! I appreciate your immediate response and concern for our well being. I am trying to remain as calm as possible, you have helped ease my mind. It's one of those things where I know deep down I am fine, but can't help get those images online of syphilis out of my head!! Thank you again.