Thank you to everyone for your thoughtful replies - I've been reading them as they appear and acting on them too, this is all extremely helpful.
I thought I'd post an update to let you know that little guy is still with us, though very poorly. I am now in that unenviable state of lurching between thinking, he's a real fighter, he deserves a chance; and the next minute I'm convinced I should pick up the phone to call the deadstock man to come and put him out of his misery.
OK so here's where we're at so far:
I'm pretty convinced he has watery mouth: he doesn't have the rattle belly but is definitely dribbling. He is way too old for this disease, being about 4 months now, which is why I discounted the symptoms before this: but it all fits. Wet mouth, distended belly and not pooing anything like as much as he should do.
This is the first case of watery mouth I've seen, and seems very untypical given his age & living conditions (outside, fields nowhere near overstocked etc). I'm wondering if he might have picked up a slow-burn e-coli infection from his mum who was the one with mastitis.
Anyway as soon as I'd figured that one out I got some antibiotics into him sharpish. I wouldn't say there has been a marked improvement - no miracle cure or anything - but he is hanging on in there and this morning I did notice he's been pooing for the first time in quite some time. So something is happening in there - though I can't hear any bowel movements to speak of which is worrying. Can't help thinking though - if he has a complete blockage in there, he should be dead by now surely?!
Advice via the internet seems to be a) feed with about 4-5ml live yoghurt and b) give him an enema of warm soapy water up the backside (not sure if this will help since he's now passing more or less normal poo?) What do you think - will yoghurt make a difference in this case?
First, the fact that he's not dead yet is pretty amazing. He nearly is: he is about as weak as it's possible for a small sheep to get and keeps toppling over, though no fitting yet. He is I think in some pain as he keeps grinding his teeth.
Am I being ridiculous in keeping him going? It's one of those things where every time I go up to see him he's turned a corner of some sort - this morning, looking a little better, yesterday morning I was really sure I should have him put down.
Damn. This one's a hard one to call. I hate the thought I might be being cruel by keeping him going - but hate the thought that I might have him put down when he's still got some fight in him. What would you do?