Why does my male cat smell the other male cat's butt?

I had a male kitten who was about 10 weeks(!) when I first saw him humping my other male cat who was a year older and neutered. The first time I saw it I was watching TV and the kitten was kneading my other cat who was sleeping, I thought how cute...but then I saw the kitten climb on his back and moving up and down and I was like ...NO WAY!!!.. so I got up and took the kitten off of my cat who was still sleeping. And when I looked I saw the kitten's little thing :O

Every time my cat was sleeping I would notice the kitten trying to go hump him, so I would say No! and he would run and hide other the bed until he thought I wasn't looking and try again. Sometimes he would even talk back to me when I said no and then run and hide. I was scared to leave my precious cat alone with the kitten in case he might try it again so I decided after two weeks to give the kitten away. Unfortunately the person gave me back the kitten. I can understand why, he has his own mind and hardly ever listens. When the kitten came back home he was again good friends with my cat but I kept them apart as much as possible when I wasn't home. I had him neutered when he was 8 months and now he is two and the other cat is three years. He has since I got him also been sniffing my cat's butt, so far I haven't seen him trying to hump him since he was neutered (but I think he would if he could). The sniffing is still going on when he is close to my other cat I would see him smell his butt and then push his nose under his tail to be closer to his butt. It really annoys me and even though I tell him no, he still goes ahead and wouldn't stop unless my male cat notice it and chase him away or he sees me getting up to move him away. So how do I get him to stop and why does he do it?