I used to get that. I realized it was because I was being pretty uncleanly with my masturbating. I would not really clean my dick off well so some of the semen would stay on my dick (under the foreskin) and have a bad odor, even if I washed it off really well in the shower the next day. I finally decided to change my habits and now it's clean and doesn't smell like anything. Also some other factor that could play a role is how hard or soft your water is. I moved and my water became much harder and could really rip away all the dirt off my body, when I shower in softer water now, it's not really like that anymore. Basically just have clean masturbating habits and if you think you're still getting wet dreams, masturbate before you go to bed and make sure you wipe your dick clean before going to sleep. Hope that helps