My husband had a vasectomy almost a year ago, but he is still saying he has some pains.?

Okay, so I don't want to get too into detail, however, I want to know if anyone else has this problem or may know what is causing it. After we have our "relations" he told me that it still hurts. Basically, right before he ejaculates and while he ejaculates he said it feels like someone kicked his balls. He also said that it goes away right away though. I don't understand why he would be feeling like that so long after his vasectomy, but he does. I feel so bad for him, and I don't want sex to become something he doesn't enjoy anymore because of how he is feeling. Any ideas what the problem may be?