I really think you need to go back to your vet for intensive hydration. They should be open tomorrow. Without nutrition and hydration he will die. Your vet should be able to instruct you how to do this. In the meantime keep him comfortable and warm and keep on with the medication.
Kate is right, he is terribly dehydrated and he may not be strong enough to recover.
I am not a vet but would think that after 3 or 4 days of medicines he should have stopped having bloody stools or there should be less blood if the treatments were working.
Perhaps his intestines cannot heal unless they are allowed to rest and this is why the vet told you to not force feed him.
A vet should know if it would be best to let his intestines rest, instead of feeding him anything and can inject him with fluids.
As I posted a few days ago, "A 50% mix of water and Gatorade or Pedialyte may help him.
You can ask the vet if there is any reason you should not do this.
You just want to be sure to put the fluids in the very back of the mouth and do small amounts at a time."
Also make sure that there is enough humidity in the air to help keep him as hydrated as possible.
These are really the only things that can be done for him.
I wish you the very best of luck with him and hope he surprises everyone.