It could be that he is not yet used to all the space. :) Try really packing it densely with plants and decorations to take up the space and give him lots of hiding spaces. Knowing that he has lots of place to hide will give him confidence. :)
Unfortunately, most of the fish that are small enough for a small tank need long footprints and/or big schools. The only thing I would suggest would be a trio of male Endler's Livebearers (they are peaceful and not nippy), but it depends on your water parameters (hardness, pH etc). Once we know those we'll know what else you can add. :) If you don't have a testing kit, your local water authority should have the information.