Husband slipped and fell on wet stairs now he has a lump at the top of his butt crack that's very painful and swollen. any ideas?

ICE... : Its very painful . Likely bruised " tail bone" or Sacrum. it is terminal part of spine.Not likely broken but X ray may be good idea. If he has new onset of difficulties to control bladder or bowel - he should seek immediate emergency assistance -otherwise - ICE it! avoid sitting , take pain medicine as Tylenol (acetaminophen) or Advil.

Fractured coccyx: Sounds like a fractured coccyx (tail bone). See an orthopedic surgeon, may want to excise it. BTW Natal Cleft = Butt crack

ER/X-rays: 22 year old gentleman status post fall on wet stairs now with swelling and pain in the lumbosacral area. I would recommend your husband visit the ER for a full physical exam and x-rays. If it is very painful there is a possibility that he may have a fracture of his vertebrae and a neurological exam is indicated.