This probably won't be very helpful, but I don't know how poop could NOT get on him during anal. I've asked women about this who enjoy anal and they always say well... yea... there's always a little. Unless you do an enema, there's gonna be poop up there. Which is also why I don't understand why guys seem to want to get women to do it. I've only done it a few times with people I was in very serious relationships with and there was poop and I was thoroughly grossed out. If I ever tried it again, I would definitely do an enema first.
With regards to your situation, it seems like this was a very casual relationship and maybe after the incident he got a bit freaked out just at the intensity of the situation and wants to back off. It doesn't seem to take much to scare guys away, if they weren't interested in having a serious relationship with you in the first place. But don't feel bad. It's probably not just about the poop. He was most likely going to move on at some point anyway.