So sorry to hear about this Dena,
but I wanted to offer my own, unexperienced and purely philosophical guess.
Chins are social animals
as I understand and it would make sense to me that if a member of the herd were sick, the others might reject it for safety's sake in the wild. I am not sure how this all applies in your case but you might consider a vet check, just to be on the safe side.
Give him a cuddle for me. Best wishes.