well im not experienced in sheep at all but you could try giving him some lactol milk it is high in nutrients, vitamins and minerals, also because its a powdered milk he may actually enjoy it, ide feed him till he doesnt want anymore basically.

i would say leave him to rest and he needs to keep as much energy as possile for when he does actually need to get up (weeing ect.) is he inside or living outdoors ??

if you brought him indoors (it will be warmer) and you could provide him with a hot water bottle so his body stays warm and he will be much more comfortable indoors.

hope that helps, sorry i could give anymore advice

p.s. you can get hold of lactol milk in most pet shops (it is for dogs, cats, kittens, and puppies) but its worth a go.