There's a remote chance that during his shipment or keeping at the LFS he got ammonia damage to his gills. I'm not sure fish can recover from this.
Maybe given time and his own aquarium with better aeration.
Mollies in general are better off with a small amount of marine salt added to their tank, if not some African Rift Lake Ciclid salts. I've never kept them in planted tanks because they really do better in lightly planted, low light tanks with fairly hard water, which it looks like your tank is set up for hard water.
Do you test for pH daily or have a drop checker? Big pH swings can be hard on Mollies
Maybe given time and his own aquarium with better aeration.
Mollies in general are better off with a small amount of marine salt added to their tank, if not some African Rift Lake Ciclid salts. I've never kept them in planted tanks because they really do better in lightly planted, low light tanks with fairly hard water, which it looks like your tank is set up for hard water.
Do you test for pH daily or have a drop checker? Big pH swings can be hard on Mollies