Starving leopard gecko?

I had bought two juvenile leopard geckos about a month ago. In the beginning they were both very active and healthy. An unforeseen circumstance landed us away from my little babies for about two weeks. I had no way of feeding them until i finally returned home. Now Mercedes is big and healthy with a thick tail, but the other one, is frail and small. He doesn't move much and when he does he just drags himself around and hardly eats. Were not sure what it is but we think he may need more nutrients in his diet or more fat. At any rate, how can I fix this problem so that he doesn't die. Time may be of the essence.