Sounds like not getting enough milk and gotten a chill. I would get him inside and warmed up and bottle feed. My 4 week old kids are currently taking 600ml 3 x day and eating hay and starting to pick at concentrates. If wet under the chin could be a sign of watery mouth vet will give you orajet or maybe something else depending on diagnosis to treat. Does he have any breathing problems cough etc. discharge round nose - If he has developed pneumonia then veterinary treatment is urgent. There could be lot's of different causes of illness, If being warmed up and well fed doesn't improve him and he is ill then veterinary treatment the only way. I would consult your vet, as young animals can go downhill very quickly. Hope he gets better soon.