Dog problem ,anyone know what to do?

So I have a very active dog. He loves walks and playing outside we often take him for late night walks, but suddenly he hates being outside after dark, he seems terrified if we can even get him out there he shakes and cries until we let him back in. We put lights on and coax him out there with treats, we got him a little light for his collar, but nothing works! He's very good about not having accidents in the house but that can't be comfortable when it's almost 12 hours. I don't know what he's so afraid of.