You said he took a biscuit, but turned his nose up at the prescribed dog
food. Try using his normal dog
food,
and see if he will eat it.
If he is not drinking or eating better, you need to get him back to the vet tomorrow.
In the meantime, encourage him to eat, and even more importantly, do your best to get some fluids into him. You could try giving him some broth, or chicken noodle soup, veggie beef soup...
Best of luck...let us know how things are going.