Underweight Platy

Just the other day i noticed one of my platys was acting strange (not swimming to well) I then noticed that he looked quite skinny in comparison to the rest .now he's lying on the bottom not swimming at all unless somethin bothers him then he'll swim for a few seconds and then go back to the bottom he looks like hea on his last legs .his belly looks non existent .it's strange because when I got them he was the most adventurous and had lots of character and nothing seemed to faze him. What could this be and is there still a chance to save him ?