Why is my partners penis sure after sex? that sore it hurts to pull his forskin back?

Phimosis: He may have phimosis, or the inability to pull the foreskin back behind the head of the penis. This can cause pain with intercourse but does not cause infertility. The treatment is to slowly stretch the foreskin so it eventually passes behind the penis head without hurting or removing the foreskin (circumcision) if this is not possible.