Ah... anal! Ok then... 😀 The two biggest fears about anal are always pain and poo - both of which can be mitigated with communication and understanding.
If you've not engaged in any butt play, you may want to start small. Lots of anal lube - I like Pjur Back Door which doesn't dry out - and get him to use his fingers, massaging all around so you're relaxed. You could incorporate toys - LoveHoney do some beginner anal dildos. My DP found that vibration helped him to relax. Again, just take your time and don't do anything more than you're comfortable with.
If you're ready to move into penetrative anal sex, it helps to be in a position where you control the depth. Either you on top or doggy with him staying very still and letting you move back onto him. It can take time for the muscles to relax. It's perfectly sensible to just wait a while for things to stretch. Sometimes DP has waited 5 minutes with the tip in before I'm ready to move! It can also help to almost "push out" as he enters you.
It's very rare for there to be poo, especially if you generally have a healthy diet and have been to the toilet beforehand. It does happen sometimes but not in great quantities! Your partner can wear latex gloves and a condom if he is concerned about any mess. Have some wet wipes nearby in case.
Another option is to anally douche a few hours beforehand. There's a video here about how to douche..
www.lovehoney.co.uk/sex-toys/anal-sex-toys/douches-enemas/p/lovehoney-anal-douche-225ml/a30997g52671.html
Finally, anal shouldn't hurt. If it does, stop and go back to basics.