STD checks

Tee
I've always been terrified of getting an STD so I'm constantly going and getting checked. Don't get me wrong, I'm 21 and only have had 2 sex partners, both of whom I dated for a while. 
​My ex boyfriend and I have recently started back "talking" or whatever. We had a lot of infidelity problems while we dated (from his part) & of course I still think about it. 
​We have had sex a few times & we haven't used protection. I've been checked & have tested negative, but I still wonder about him. Can I ask who he's been with sexually? When was the last time he's been checked? Does he use Protection with each girl?? Suggestions? Comments?