We had some people who went through our property to get to a job site. Every day they threw out their trash as they drove through us. I finally got sick of it and I started picking it up and dropping it in a pile at their job site while they were not there. They caught on very quickly and stopped littering without a word being spoken.
I don’t know if this would be the right tactic for you to use but it sure worked for us.
Tried that. Didn’t work. And neither does he................