I am assuming he is only smaller than the others and has no other problems. In my experience, I have found some lizards to be "runts"if you like. I have had bearded dragons, water dragons and frilled dragons grow at surprisingly different rates when exposed to the same conditions. I guess in the wild these would be food for some other animal. In captivity, they still make great pets. As long as it continues to feed, it should continue to grow. When I had a smaller beardie, I spoon fed it baby food, favourites were fruity apple and pumpkin and sweetcorn. When she got used to this, I added finely chopped up fresh vegies. I fed her this every afternoon outside and she soon started to catch up. I also gave her crickets in her enclosure. Now, about 5 years later, she is one of my best eaters. I have also used a product called Herpaboost on an eastern water dragon with success. This product claims to stimulate juvenile feeding.