Did he have the leukaemia included cos if so its probably that.
What your vet says is the same as what mine said when Misa was ill although it was the second part as he had the first while still in rescue.
It may take a few days for him to feel good again but definately take him back if he still not eating and a bit better to morrow.
Misa perked up a bit by 24 hrs but it then took about a week before he was back to normal. My vets had said if he wasnt better in 24 hrs they would take him in and put him on a drip.
I hope Charlie feels loads better tomorrow