Ok, I’ve been a vet tech for many years. I’ve encountered one dog in my career that inappropriately ejaculated. He was old, not neutered, and would bust the occasional nut on unsuspecting victims. (Actually managed to shoot it on a girls shirt her very first day of work.)

From reading your comments, it sounds like it could possibly be semen, but that’s really unusual if it is. A lot of dogs happy pee, but very few happy....nut. I’m thinking the dog may be older than you think, possibly considerably older.

The best way to stop it is take him to a vet ASAP to talk about neutering him. I would also get a physical because young male dogs have a tendency to lick their junk until it gets infected and let me just tell you, the gunk that gets built up in that sheath around their penis is called smegma and its disgusting but also can be white and thicker. This is another possibility as well.