It takes them awhile to adjust. It took my first one 2 weeks to get over being shy. My second Beta took just over a week to get comfortable. That was in heavily planted tanks. He probably feels to exposed and the movement outside the tank could be stressing him. They have pretty good eye sight. Put a background behind the filter. Something as simple as a piece of black construction paper would work. Add some plants, real or fake. If fake consider getting cloth ones with no wire in them. You have to change them every so often because they get dirty but they will not hurt his fins. Hard plastic ones could possibly damage him. Some Betas do not like any water movement on the surface. You could put a piece of plastic canvas in front of the outflow to break it up.
