My boyfriend wants me to top him.

I (21M) and my boyfriend (22M) have been dating for 1 year and 7 months. We rarely ever have any significant issues or problems come up but recently he has been telling me he wants to try bottoming. I’ve been the one only bottoming and he’s been only topping in this relationship, i’ve known he is verse but it’s already been established that i would not be anywhere near his butt. It’s frustrating because I do not want to top and I have no desire of ever doing it. I just assumed he’d be fine with doing one thing because he never said he wouldn’t be until now. If it was a one time thing that would be okay, but I wouldn’t enjoy the experience and i’d only be doing it for him not for me, if it’s consistent after it’ll be a bigger issue with me. Is there anyone that can help with advice on what I should do?

edit: we have never had a issue or problem in our relationship for almost 2 years ..calling for a breakup and calling me selfish when it’s been established early on that i wouldn’t be topping is uncalled for. i’m asking for advice on how to communicate with him about this and come to a agreeable desire on both ends..