Balls hang high or low depending on their temperature, they can crawl up at the point of orgasm but they quickly descend other than that, the way they hang has nothing to do with sex.
They say most of the time if you're suspicious it's for a good reason. I don't know him or you so I can't say. Getting semen on your shirt is very reasonable when you masturbate, you never know where that stuff can end up.
If he's not cheating on you but you keep accusing him of it, he's going to think about it. Make up your mind, is he a keeper or not. These feelings didn't come out of nowhere, other things have happened to lead down this path. Talk to him, don't accuse him just talk calmly about what the 2 of you want out of this relationship and see if you both really want the same thing.