It really depends on whose telling the truth in this situation but either way if the ex-wife has an STD, regardless of where she got and even if it WASN'T from him.. there's a possibility that he has it too which means there's a possibility you now have it as well.
I'd hold off any further sexual relations with this guy until you know for sure what he has or doesn't have and if you have or don't have it as well.
If there's any truth to what he's told you about his ex, the likelihood is high. Also, it is a possible to get pregnant while on your period... whether the possibility is great or small, there is a chance. And, be aware there are men who will claim to have had a vasectomy when they, in fact, hadn't... so they can justify having unprotected sex and to deny paternity. For your own protection, and until you know this man better... why not use condoms?
First of all doesn't matter who had sex with who you never trust anybody who wants to sleep with you when they say that they caught diseases on the type or treatment or when they say they are on any type of birth control you haven't seen.