Goodness, that picture really should have a 'would you buy crack cocaine from this man?' caption. He really doesn't look like a pleasant character.
Must say, I'm lost in admiration at the power of his sphincter muscles. Holding all that stuff in for 4 days.
I agree with you OP it does look very odd to read the word bum in a newspaper. Surely rectum would have been better, or even arse at a pinch if the writer was finding it hard to come up with an appropriate word.
The article is in a newspaper so presumably aimed at adults, so use an adult word. Bum is sort of rude but somehow twee and childish at the same time.