So Gus is sick...

My rabbit is sick and there are no vets open til morning (it's coming up midnight where I am). I was hoping the people of r/rabbits could help me?

I got home from a shift at work about 2 hours ago to find diarrhoea on the rug and one very poorly looking bun. Gus is about 3 months old and was right as rain when I left him early this morning. Some time in the ~12 hours I was gone he's been ill. No change in diet, nothing I can think of that would have brought it on, and his partner Mrs Nesbitt is fine. There was an alarming amount of poo, and Gus is very lethargic. I got him to eat a bit and drink slightly sugary water (he wouldn't take plain water), first off my fingers and then from the bowl. His energy seemed to pick up a little after that but he's still very off. He seems to have lost a lot of condition and feels much thinner than he did before.

I know I need to get him to a vet ASAP but unfortunately ASAP is a while away. Just wondering if anybody has any idea what might be up and any advice for rehydrating him and getting him through the night?