Thank goodness I had some Critical Care on hand, but...

...it's not coming out the other end looking any better than before.
Mills has been going off his hay and looking rather skinny. The vet couldn't find anything wrong except for a tooth that was growing a spike. It seemed as though the tooth HAD to have been the problem because nothing else was found that could explain why he'd go off his hay.
We got home and it just got worse! His poos were quite small, hard and black. He would eat his veggies and some pellets but still no hay. I ran out and got him orchard grass which he seemed delighted by when I put it out for him, but again....he just nibbled a piece or two and left it. By this time it was evening and our bunny clinic was closed.
Mills is not quite as energetic, but still is in good spirits. I wasn't going to wait any longer for it to get worse so I got out the Critical Care and started feeding him myself. He has been taking it like a champ! So the good news is that I'm getting food into him! What I'm wondering is how long will it take to come out in bigger, healthy poos? His poos are still small and dark, but not AS small and dark as they were. When would it be time to panic and go back to the vet?
It's crazy how much time I spend worrying about bunny poo.:lol:

Can anyone tell me how it should take to get some decent poos? Please?

I assume he had a Dental ?

Sorry to hear your bunny has been a bit off colour. I've had that before with one of my bunnies where they have gone in to stasis and the only thing the vet could find wrong was a little tiny spur on one tooth.

Did he go off his food altogether at any point, or was it just the hay?

I know when Starbuck had been treated for stasis it took several days to a week until he was back in a normal eating pattern. And then I always think that healthy poos follow after that...but hopefully someone with more knowledge/experience can say more about it.

Is he eating hay again now?

Yes, he had the tooth trimmed. No he's still not eating hay. I have been feeding him critical care for over 12 hours and the poos are getting worse and worse. We're going to the vet.

Is he on pain relief? Did the spur cause ulcers/sores in his mouth? How long ago did he have the dental? Was it under a GA? Good luck at the vets.

Mills isn't on pain relief - he doesn't seem to be in any pain at all. The only thing he's done is go off his hay! It's so weird! He has an appetite but his poos still aren't looking like they used to, but are closer to a normal size than before. I suppose having only veggies, pellets and Critical Care in his diet would make smaller, darker poos than a diet of hay instead of the CC.
He has no sores in his mouth. He didn't have a GA or any other meds to have his tooth spur trimmed, which was 1 week ago. We have been back to the vet twice since then and his tummy feels fine - he's actually gained a bit of weight since his tooth was fixed. But he STILL won't eat hay!
I am a nervous wreck just waiting for something horrible to go wrong. One of my other buns has a scheduled dental on Wednesday, so I will be taking Mills along for another check...unless we wind up having to go again before then.

Do you think there is any chance it could be a problem with the hay? Is it nice and fresh? Just throwing things out there off the top of my head...

You could try a new batch and see if that helps. Does he eat grass? Or trying a different type of hay, maybe a little bit of green oat hay or depending on what you feed, a more stalky type? Rabbit safe twigs and branches might also help if he is interested in any of those.

Glad to hear his poos are getting better though.

He'll need fiber to increase his poop size.
Without any sores in his mouth it's possible hat he's trying to make up weight & neglecting his fiber intake (hay)
An alternative if you live near the country, is to see if he'll eat bramble (wild blackberry) leaves or hawthorn leaves. The poops will stay dark cos they're green but should increase in size.

He wouldn't eat even the softest of hays, but we did finally find the problem yesterday!
He has a tooth root infection. I figured it had to be a tooth problem - why else would he go off his hay? - but his teeth and jaw looked just fine until I took him in for a weight check yesterday and found he had lost 100 grams in a week!
Now, Mills is on Critical Care twice a day, fresh veg and some pellets and has has his first penicillin shot in a series of 6.
I'm so relieved to finally know what's wrong and be able to help him!
Thank you everyone for your input and suggestions. RU!