Here's a tip, if you have a letterbox, nail it up and put one outside on the wall. (you can get them from hardware shops, Focus). We did that years ago when I taught martial arts and all sorts of weirdos put things in my letter box. It's easy to do and stops anyone putting things through your door. It's a start at least. Without any witnesses its easy to put blame even tho you think it was probably him, it might not be! Can you set up a video in the window discreetly at all? worth it if you can.