I want to know if I'm at risk for AIDS so I'm going to ask my ex about his sexual history.
Out of curiosity, is there a reason why you are going to do this?
I want the insurance of knowing the truth
It's reasonable to ask a sexual partner's sexual history and STD risks and typically it should be done before starting a new relationship. If you think there is a reason you are at risk, you should also ask your ex. Out of fairness, they might ask you to provide the same information to them.
What I do with my bodily fluids is my business
I support you asking your ex, but ultimately what you need to do regardless is go to the doctor and get tested for all STDs, including AIDs. Your health is important and your ex might not be willing to share everything with you at this point.