First time anal advice

So this guy that I'm kind of fwb with is an ass guy and really wants to have anal, which is fine because he's super fucking attractive and the 🍆 is totally worth it. My biggest fear though is not the pain, but the possibility of pooping on his penis. Like what if it just happens? What if you don't have to go before but then it just happens? I've already thrown up on his penis because he was literally humping my throat and my gag reflex is the freaking WORST! If this happens, there's no coming back from it😂😂 So I guess I'm just looking for a little first time advice and some tips and tricks on how to make it less stressful and more pleasurable for me, because he is well endowed and I'm also kind of worried about the pain. Thanks in advance!! ☺️☺️