With creatinine so high, the fluids are the best option to getting it down. Let me know ...
What weight is your dog?
What type of SubQ fluids are you giving?
How much (dose) SubQ fluids are you giving?
How often are you giving SubQ fluids?
Also, your dog needs to be on a kidney specific diet for now. This means low sodium, very low phosphorous, high quality proteins (human grade) ... the easiest way of achieving this is to use a canned kd dog food such as Hills kd.
Do you know what phosphorous blood levels are?
If he's still eating okay, that's a positive sign.
The problem is it really depends how much damage has been caused to the kidney organs. That will determine whether they are able to fulfill their filtering role to an adequate level, with or without further intervention.
Has the vet undertaken a blood pressure check? If not, it needs doing.
Get back with answers when you can.
Tony