I believe the cat's rear end is an erogenous zone. They get excited when you hit them there. Lots of cats seem to like it when you really give them a good slapping there, sometimes even hitting them with objects.
Two of my three cats like it when I spank them. The other just complains and runs off.
Cats are individuals. You may find that this cat loves it and the next cat you meet will tear your face off for trying it. It's really one of the most endearing (and frustrating) things about cats. You can't say "all cats" this or that. So you get to learn about your specific cat over the course of his lifetime. To make things interesting, your cat's tastes may change with time. As he gets older, he may not like this so much.
Both of my boy's love being patted there, one so much that when my youngest boy tells him come on for a massage he jumps on his cat tree for him to pat him.
Feeby has now made what I call 'butt pats' a morning ritual. I think it first started with me giving her a couple of pats on the butt thinking she wouldn't like it and would stop bugging me while I read the news. But, she loved it, and now - without fail - she eats some of her breakfast, comes out onto the patio where I am, and stands there beside me waiting for her butt pats.
But, it goes further than just that! I tap out the rhythm to a child's song on her one side, and finish with a "Shave and a haircut, two bits" to let her know I am done. Then, she turns around for a repeat session on her other side. Once both sides are completed, she walks off to eat some more food.