Leaving him outside alone in the evening is really a bad idea, especially since a chihuahua is basically an appetizing snack size for a hungry fox. That said, I've seen foxes in the green areas around Milan and they're not as aggressive as people think - they look for easy food (garbage, mice) and if they hear your dog barking and moving around, they usually prefer to stay away. The real problem is when they're alone, quiet, or in open spaces where escaping becomes difficult. Keep him inside after sunset, take him out on a short leash if you need to go out, and maybe install motion sensors if he has an outdoor space - foxes hate sudden lights.