How long you have been having IUD?
Did he start to complain about the sex from the first day you inserted IUD?

The IUD should not be scraping him, it is likely the STRINGS.

You should have it looked at to see if the strings can be trimmed, or to see if the IUD itself is displaced and down in the cervix.

I have an IUD and once when my husband and I were having sex, something didn't feel right to me and when we were done and talking, he agreed that something didn't feel right. I reached in with my fingers and found that when I pressed against the strings, I felt that feeling that didn't feel right. My solution was to push the strings into a different position. It hasn't been a problem yet. If your strings are at a length and position where you can do this, do it. If they are too short then I don't know what to tell you. If you can't do it for another reason, I also don't know what to tell you.

They might be long enough where you can tuck them up around your cervix, or your doc might be able to trim them to a more comfortable length.

Yes it is more than likely the strings, they can feel very sharp at times. Because they are like a plastic material almost like fishing line not a real soft cotton string like a tampon.