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He may need another dewormer, his first round may not have knocked out all the parasites and by the sound of it he may have a parasite. The voracious appetite and vomiting and diarrhea all sound like a parasite issue, coupled with the fact that he is not gaining weight like the others. I would take him to the vet ASAP so they can do an exam to verify the prescence of a parasite. They will need to check for all parasites including heartworms because these can cause vomiting as well.
Hope this helps!