You are a lucky girl and should keep the honest and open communication going.
I would focus the conversation on topics like enjoying his vulnerability and how him being on verge of loosing control is a huge turn on for you. You want to understand how all parts of his body functions and reacts to a variety of stimuli. You want to engage him in multiple ways, simultaneously, to see how he is able to multitask, and where he starts to loose his concentration.
You can very clearly explain that you are not interested in poop. But with proper health and safety considerations, you are interested and turned on by anal play for both if you, and you enjoy exploring new things with him. You are not afraid to get up close and personal with his ass. It might sometimes be with your tounge, but more often it will be with you fingers, toys, or appliances.
I love the idea of you being in the bathroom with him when he is pooping or expelling. You may start with kissing him while he is trying to go. Work your way up to sitting on his lap naked, facing him, and masterbating while he sucks on your nipples. All the while he is also trying to use the toilet. You will not stop or get up until you hear him go.
You can't get rid of a fetish. It's what you are naturally curious about and it turns you on. You can't just choose not to be turned on. Enjoy it and work with him to explore it. Also, I recommend you offer up your own body for him to explore in similar ways to satisfy and promote his curiosities. This way you both can learn how it feels to be on the giving and receiving ends of these activities, and it should give you more ideas to explore.
Please keep us posted on your progress and fun activities. We love to hear about it all, and it's how we learn things too.