Continuous Male Orgasm

I'm so worried about my husband. When we have sex and he finally ejaculates.. he can't stop the very intense orgasmic sensation that happens when he is ejaculating. It can last up to 5 minutes. Its weird because I am the only woman that he ever slept with and that that has happens to him. At first it was a turn on for me but now I feel sorry for him because he gets so exhausted because he constantly feels like he is ejaculating after he actually does. Should I be worried? Should he see a doctor? Why does that happen?