This might be because he has a sheath infection, a bladder infection, or just because he was aroused by the scent of a female.
I would wash it off and see if it comes back or drips more.
He could have a hair or weed seed inside his sheath causing infection.
If it keeps up or becomes more obvious I'd see the vet as he may need the sheath cleaned out or some antibiotics to resolve this.
Hope this helps you!
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