I laughted a lot reading your message. Because it sounds like your husband just likes to spank :)
Spank sounds like a kinky way of enjoying your slap. Especially if its to buttocks.
I am a person who was born muslim and i have never heard of spanking. However there is slapping. I am not a scholar or something but in our culture, its ok to slap your wife only when there is a huge issue and there is no any other solution then breaking up. Slapping is the last option to try to keep the relationship.
I am 99% sure it doesnt say anything about spanking on the buttocks anywhere in Quran. But i remember there are some parts in Quran that is taking about slapping your wife and punishing her for specific reasons.
Therefore i would personally talk to the husband to clarify this action and its reasons. Spanking on the buttocks is not an Islamic way of punishing your wife.