Can you get a picture of the sore on the frog's belly? Since there's no way that won't get bedding in it, you should switch the bedding from coco-fiber to moist paper towels. These need to be changed daily.

The sore, and the purple/red colors you described makes me think there's probably some sort of infection going on as well. If that's the case usually they'll need antibiotics. But deal with the toxing out first. Until that's taken care of there's no way to tell what (if any) symptoms are due to the open sore, and what's because he's toxing out.

For the open sore, you can put original Neosporin on it. The kind WITHOUT PAINKILLER. The kind with painkiller will kill the frog.