Poor little thing, you have certainly done all you could have for him. Sometimes they do survive against all odds though. My little Gotland Tiny, the one who was a whisker away from dying earlier this year, was not expected to make it by the vet or the people at the agricultural merchants where I asked advice. But, every day despite the fact she could not stand or even sit up without being propped up via bales, she was eating (albeit not very much), and I thought while she was eating there was hope. Gradually she rallied and is now amazingly plump and healthy looking, and is fast catching up with her peers size-wise.
So I reckon there is every chance yours will make it
Blimey woolleyval I too have loads of them in my fields (and nesting in my chimney!). My lambs are all sprightly and alert so hopefully they aren't going to be crept up on! Horrible things ...