As far as I know, sperm does not smell unless it's starting to dry up. If you wipe it off within a minute or less, it should not have any scent. If it does, it could be stagnant sperm. meaning, it was produced earlier but not ejaculated and it sat in his sack for a while, fermentating. You think I'm kidding?
I'm serious. If his freshly ejaculated sperm smells bad, there could be something wrong with his... spermozoids.
Unless you have a dog's nose(like me) hehehe.
Try it, you might like it. Just plug your nose and breathe with your mouth, but close your mouth when he's about to .. you know.