My runts only lived 3-5 days and never grew, in broiler chickens with Clostridium perfringens (necrotic enteritis) subclinical infections and those that survive it often suffer damage to the gut and don’t grow as well or gain weight resulting in economic loss if spread across enough birds, if it is only 1 bird affected in this case I’d lean towards developmental issues on his part rather than an infectious, toxic, or nutritional cause, I had 20 percent death loss in two genetically separate batches, all unaffected birds growing normally. I initially had 1/6 die and thought it was just him, but a month later I had 5/24 doing the same thing. Developmental issues would be on the top of my list unless you start noticing the same pattern in more birds, nurse him along and see what happens, my wry neck chick is certainly spunky! Won’t breed him but as long as he can survive on his own I’ll let him!