I don't know anything about African frogs, but I used to breed linebacked woodfrogs as a hobby, and if I had to take a guess, I would say it's either a fungal infection or a parasite. It's hard to tell from the picture. I am probably way off.
In any case, I hope it's nothing serious. Is his skin stretched over the spot, or is the spot ontop of his skin? Has it grown at all?