Hernia is a condition in which the intestines protrude out of the abdominal cavity through some of the holes in the abdominal wall. In hernia these holes are wider than usual.
Inguinal hernia is a type of hernia where the intestines leave the abdominal cavity via the inguinal canal into the scrotum. The inguinal hernia itself can not affect male fertility, except if the intestines put too much pressure on the spermatic cord, thus increasing the chance for the testicles to get swollen. However, this condition is accompanied with pain and your husband would have recognized if one was already present.
You might want to have his sperm tested with spermogram in order to determine the actual quality of the sperm and to rule out possible disorders.