Need help urinating during sex? [erection]

ive been trying to urinate into my boyfriends anus lately. the goal is to piss into his hole so he can fart a golden shower over my face. the problem is i can never urinate with an erection. we've considered having me pee into a jar before hand and then pouring into his hole during sex, but we both agree this would ruin the intimacy. does anybody have any tips on how to blast piss into a dudes ass without losing an erection?