Would it be irresponsible of me to not ask my fiancée if he ever checked whether he may have contracted HIV or other sexually transmitted diseases from previous sexual relationships which he revealed to me?
It is wiser to pursue your concerns now. This is your only opportunity to inquire before marriage. If it turns out that he is HIV positive or has any other STDs, then you will have to take excessive precautions in order to maintain a healthy marriage in the future or choose to dissolve the engagement now. He should be just as concerned as you are and may have already addressed the issue. You are not doing anything wrong by respectfully asking him what is responsible and fair for you to ask.