In nature, there are always a few "runts" - especially so in the case of frogs and tadpoles. His slowness to gain strength may be due to a low tolerance for the slightest problems in his environment, or he could simply be one of the weaklings that are usually picked off early in the wild. You could turn on red or black lights so that you can monitor feeding and have him feel like it's fully dark. A few pictures of his weight and stature would be much appreciated. I'm sure that with care he'll gain strength. Also, how large is the tank they're currently in, and are you planning on keeping all of them together?