Can Somebody Help Me Save My Poor Betta?

Hey all,
My betta does not look well at all, and has been very lethargic. It looks like is is losing a lot of scales/is covered in sores . I am having trouble diagnosing what this is. Can somebody help me identify what is wrong with him and give me any advice on treatment thanks! I will also include pictures.

How long has the tank been set up for?
How big is the tank?
How often and how much are your water changes?
Have you tested the water?
Have you added any new fish to the tank?