Topping is when you put your penis in another guy's butt hole during sex. Some guys may think they cannot get HIV by topping. While topping is lower risk than other things you can do in the bedroom (or car, or park, or wherever else you like it), there is still some risk involved.
For HIV-negative tops, HIV can be passed from your guy's butthole through the head of your penis if you are not using a condom. Guys who top may be more likely to get HIV if your guy is HIV-positive and not on treatment. Maybe he doesn't know his HIV status? In this case, he could have a high viral load, making it more likely for him to pass on HIV. Having other STDs, like gonorrhea or syphilis, can make both of you more likely to pick up or pass HIV.
Condoms are the best way to prevent HIV, even if your guy has an undetectable viral load. If he is HIV-positive, taking his medication as directed and getting tested for other STDs will reduce the chance you become HIV-positive from doing it without a condom.
Tops who are HIV-positive are at risk of passing HIV to an HIV-negative bottom if they are not on HIV medication and are not using condoms.
Taking medication as directed by your doctor is important in having an undetectable viral load. An undetectable viral load means there is less of a chance you can pass HIV on to your partner. It is also important to get checked regularly, in your butthole and throat, to make sure there is no other STD that could be increasing your viral load.