, and welcome to the forum!
Just a couple of quick recommendations for you: If Jasper were here, I would be getting some food into him immediately. Cats cannot go without eating close to their minimum calorie requirements for very long...before they begin to develop a very serious liver disease ("hepatic lipidosis" or 'fatty liver disease').
If you can, pick up some of this food from the Vet's office: "Hill's a/d"". This is the one: a/dŽ Canine/Feline Critical Care - Canned
This will be the easiest one to use for now, it's high in calories, it flows through feeding syringes very easily.
"Force feeding?".....you are going to have a much easier time of this if you get into the mindset
...you're helping, NOT forcing - this may seem trivial but, believe me, it can make all the difference - for you, and, for Jasper.
You must be extremely careful that Jasper does not aspirate - breathe in - the liquid food...if that happens, there's a great risk for another serious condition (aspiration pneumonia). So, little drops at a time, put into the sides of his mouth, he swallows, a few more drops. NEVER squirt/put anything straight down a cat's throat.
There's lots of info on this page AssistFeed.com: Advice to help a sick cat who will not eat
A quick video course here
If you need more help, there's a group of people online 24/7 here Feline-Assisted-Feeding : Feline-Assisted-Feeding
Now, you asked about "other renal foods"........for now, don't worry about them. This Hill's a/d is close enough to the 'renal food requirements' that it pass for now. The fact is, there are many other foods that people safely use for kidney cats....many people ditch these "prescription" diets and use selected high quality alternatives. But, again, for now, the a/d would be perfect.
If you can't get a/d today, see if you can find a can of Wellness grain-free canned Turkey: Wellness Turkey Recipe
and, using a blender, turn it into a smooth, slurry, "thin" enough to go through the feeding syringe.
It's very probable that he will have excess stomach acid - there's a "human med" you can safely use for this: Pepcid AC. Here's some info about this: Tanya's Comprehensive Guide to Feline Chronic Kidney Disease - Nausea, Vomiting, Appetite Loss and Excess Stomach Acid
Hopefully, there's enough here to get you started and feel a little more at ease.
Dealing with kidney disease is very much like a balancing act - the natural balance of things is skewed...our job is to make interventions that help re-balance their systems.