There are ways to avoid any risks of anal sex but things need to be taken very slow. Injury happens when a penis is repeatedly inserted through a contracted sphincter, which then causes tears. The more tears you have the higher your risks for incontinence (which, isn't really that common).
So before you start make sure you have lube, condoms, and if fingers are involved then make sure the nails are clipped. When you start make sure you apply lube to both the penis and anus. When he enters have him go slowly and right when you start to feel discomfort then have him stop but not pull out. Within 30 seconds or so your muscles will tire out and relax. After that you should be good.
Try a position where you're on top so you can control it a little better and after you get the muscles to relax then you can do other positions.