He doesn't even have HIV as far as you know! Just because he's promiscuous doesn't automatically mean he is HIV+.
In an adult, only unprotected anal or vaginal sex and sharing IV drug needles with infected users is a risk for HIV.
Therefore, don't share drug needles with your roommate and if you have sex with him, always use a condom in order to avoid HIV. Touching surfaces that he touched, using the same utensils, sitting on the same toilet seat, drinking from the same glass, none of these or anything else you can come up with are a risk for HIV.