I agree, have a vet take a look at him. Even if you can't pay, just bring him in and ask about cost and say that you are just trying to help the frog. Try catching it with a net. Do you have a vet school in your state? It would help to know what state you are in.

An infection can be fungal, viral, bacterial, or a protozoa.

If nothing else, I would catch him and put him in an aquarium. I really don't think he's going to get better, if he's been going down hill on his own.

Small crickets might be a good start for him. Dechlorinate any water that you use.

Good luck, please keep us update!

Sincerely,
MasterHoyt