Sounds like he may have fatty liver disease. Classic symptoms are not eating/drinking, weight loss and vomiting. If he doesn't have it, not eating for any longer will cause that. so you MUST get food into him. The syringe feeding is the best way.... as much as you can until you get him back to the vet.
Mix wet food with warm water (blend if possible) and syringe 1ml at a time into his mouth pointed slightly forward from the time so you don't aspirate the food. This may cause him to vomit more but as long as the food is into him for awhile he will get nutirents.
Did the vet check his skin colour?? Check the skin by his ears.... not eating can cause rapid liver failure in cats and his skin will be yellow...... if he is at this point he HAS TO HAVE FOOD, and he may even need an emergency vet to get a feeding tube into him.
Also did the vet give him any fluids/ IV or otherwise? This is very important and can kick start them into eating again/feeling better. If not he probably needs fluids ASAP
Edit: I see that he did get fluids, but I agree he may need more before Monday. Syringe feeding will get fluids into him as well as food but often it isn't enough. The only reason I know this is because kismet went through this, lost over half her body weight and I had to syringe feed her for weeks and give her subQ fluids to save her life.
Good luck and wishing you tons of good vibes