See images below. We're not sure of the cause, or cure at this point. He's trying to eat, but can't seem to open his mouth. He hides in the caves a lot, but we know this breed will do this...
Not sure if it's mouth rot, or what. Any ideas? Would this be fatal?? No other issues w/other fish in the tank, though we did just lose a female guppy a week ago (no symptoms, just expired).
It's hard to tell from your pictures (I'm not blaming you; fish are a nightmare to photograph, we all know that!), but it looks like either a tumour or a dislocated jaw.
I know it's really difficult, but of you could get a better picture, or give us a better description, that might help. Particularly note any redness, white patches or 'fluffiness' to the growth, or any cauliflower appearance, as that might help the members here with a diagnosis.
His mouth does have redness to it but it is also white. He has started to hide more and it seems to get worse everyday. If it was a tumour or a dislocated jaw is there anything to cure it or will he just die? Sorry for the pictures again but it is hard to focus with water and he keeps moving.
I'll be honest; I don't think that's treatable I mean, we could suggest an anti fungal, as it looks like it has fungus as a secondary infection, but that won't help with the underlying issue, whether that is a dislocation or a tumour.
It might be best to euthanise the fish, rather than leave it suffer, but that's a decision only you can make. Let us know if you want some advice on how best to do that.
I'm really sorry I can't suggest anything positive
I mean, we could suggest an anti fungal, as it looks like it has fungus as a secondary infection, but that won't help with the underlying issue, whether that is a dislocation or a tumour. 
