So we made another trip to the vet Saturday morning. Diesel is now scheduled for ACL repair surgery this Friday
She said it would heal on it's own, eventually, but based on how slow it's going it could take up to a year to scar up properly. With how much it still hurts him there is no way I can let it go on like this.
The plan is to sedate him Friday morning, confirm that it's his ACL and perform the repair. He'll get a "cone of shame" while he's still under anesthesia (because I have zero chance of getting it on him once he's awake!) and he'll have his whole leg wrapped for about 7 days. He'll have to stay in our giant dog crate for at least those 7 days possibly a little longer depending on how it's healing.
On the plus side the vet said that cats have an almost zero reoccurrence once they've had the surgery. The material they use is the same as what they use in dogs and since cats are so much smaller it is very unlikely to fail.
My poor boy has been through so much in his short 2 years, I really hope this is the last thing he has to endure for a very very long time.