It's difficult when the prescription says take the medicines with food, but your dog doesn't want to eat anything at all.
At that point I would be inclined to call the vet for advice. The most important thing is getting fluids down him. I found with my dog that if I used a plastic syringe with no needle, and used water with a little honey and a pinch of salt (or an unflavoured electrolyte mix with water), then syringed small amounts at a time into the corner of her mouth I could manage to keep her hydrated.
I found the best place to do this was just in that little gap between the molars and the canine teeth. They swallow as a reflex. Not too much fluid at a time, but repeat often.
It is days ago since you posted this, and I missed it until now. How is your dog doing? I hope there has been some improvement. Did you call the vet for advice?