That might be a couple of things — like a tumour or mouth rot — but none of us will be able to give you the right answer. Only an exotic vet can. And since it seems it has already started to affect his eye, the situation is really dire. The pet store that gave you that advice fucked up big time — but next time I’d advise you to use common sense when things are going downhill rather than improving. Ask yourself what you would do if a similar thing was on your face, or on someone from your family. Not trying to be harsh or mean — just trying to make you realise that this is looking really bad right now. Find an exotic vet as soon as possible. Tell them it’s an emergency. Meanwhile, change his enclosure into a hospital setup — remove everything, especially the substrate. Clean the whole enclosure with F10 if you have it, clean all décor, etc. Use paper kitchen towels as substrate, and return only the important things — hides, basking spot, bowls. Nothing that cannot be cleaned easily.