Any help/advice re my sick dog?

Scooby is 2.5 yrs netured boy. Generally delicate so I have had a few hiccups caring for him but generally he's been healthy and happy.
Wednesday he refused his dinner. I slept downstairs with him and in the night he ate some grass and threw up some grassy bile. Wouldn't eat or drink after that so we went to the Vet Thursday. General exam revealed nothing. She gave antacid and anti emetic sent us home with rehydration drink and antacids. Thursday evening passed a stool with digested blood in it. Friday still no fluid taken so back to the vet. Vet did blood tests and x rays gave antbiotic and admitted Scooby to put him on a drip and observe him.
Discharged him Saturday but he was depressed and lethargic within an hour of coming off the drip. Took Scooby to emergency Vet who did more blood tests and pancreatitis test which revealed nothing. His body temperature is low, his urine is very dilute and he has abdominal tenderness. As of this morning he still will not eat or drink although the Vet nurse is putting food into his mouth and he is swallowing tiny amounts.
I am desparate for a diagnosis - does anyone know what may be wrong with my poor boy?

I am so sorry Scooby is unwell, sometimes it can just be a bad tummy bug that needs time to heal, you've done the right thing by keep bringing back to the vet, I hope he gets better soon :flwr:

As I started reading, I thought maybe pancreatitis, but then I saw the vet has tested for that, so not sure... At least he is safe at the vets and hopefully it is just a bug that will pass through. Healing thoughts to Scooby!

Thanks for the replies - it has been a difficult few days
I got him home late afternoon yesterday and he is much better. Still no diagnosis which is concerning as I want to know why he was so ill. I was up in the night with him twice and he has loose bowel movements but he is eating and he drank all the water in the Cats' bowl.
My tabby cat was so pleased to see him - she kept headbutting him and sitting by him. Scooby chases and humps the cats (despite lots of training and positive reinforcement if he behaves well) so it was unexpected that the cats would be glad to see him back.

I think maybe it would be worth seeing a specialist -- eg at a vet school. Often pancreatitis is not diagnosed on single tests. Vets can only take things so far in the search for a cause, just as GPs can only do so much before they refer people to a specialist. If he doesn't make a full recovery now I'd be looking for a vet school referral to someone with a speciality in the area. Hope he continues to do well! :flwr:

Scooby is recovered and is eating and drinking well now. He seems a little lethargic but that could just be down to our lovely summer !
He was diagnosed with a urinary tract infection the day after the Vet discharged him and is on antibiotics for that. Vet doesn't think it was the UTI that made him feel so poorly but a symptom of his not drinking for five days. The claim for the Insurance says Anorexia leading to UTI. I guess I may never know exactly what made him so ill but its really good to see him happy again.