Several things could be wrong. Being housed together with your bigger dragon, most likely the larger dragon is getting the majority of the food, stressing the little one, taking the best basking spot etc. ALso, having an injury will also stress that dragon and his body would be putting more energy into fighting infection and healing the wound.
My suggestion, separate all your dragon babies into separate containers, feed them healthy greens and well gut loaded insects. Make sure all uneaten crickets are removed before lights out to prevent the crickets making meals out of your bearded dragons. If you can't be sure all the crickets are gone, then at least leave greens in the cage over night so the crickets will have something else to eat and drink other than your dragon.
If he doesn't start to grow in a couple weeks, or slows down eating, the bite gets worse etc, then get him to a vet and/or get a fecal done to check for parasites. (if no growth but eating well, wound is healed, or dragon becomes lathargic, might be parasites).
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PHLdyPayne