My guess is he is trying to mark the same spot as Samson...not Samson himself. My pups all do that...they see one peeing and they run over to go in the same exact spot. The girls and the boys!
OK...editing this...as soon as I posted, I came up with a solution. Joking of course...but have to say it. Put a raincoat on Samson when he goes out.
I hope you have a sense of humor.