Awe, what a little cutie!! He is definitely undersized for his age. But it sounds like he packs quite the attitude in that little body! It can take them a long time to recover and rebuild their fat stores. A fat cell will never completely go away once it has been developed, but if it has been depleted it takes awhile to build it back up. When Grif was sick and we had to force feed her it was over two months before she looked "healthy" again (which in pacman terms means chubby). It can be hard to tell exactly how much of a frog's mass is fat and how much is water. Most of the time when you see a pac bloated looking it is due to water weight. Keep up what you are doing. Get those calories and vitamins in him. He needs the nourishment. Keep a close eye on him, but it sounds like he will be fine. Again, he is adorable!