good job on husbandry! i would just may be offer variety of food, rather then just roaches.

it would be great if you are able to take a better pic, cos right now i can't see a thing clearly, it can be anything, infection or parasite or something more serious or it may be nothing. I would not suggest to treat for anything based on those pics.
melafix won't hurt even it is not bacterial, but i would place him in hospital set up until healthy. if you fed him bullfrogs, you need to go to vet to get fecal test done asap.