Great. Thanks for the additional information. I really appreciate it.
This really does sound like either a completely or partially torn CCL.
The CCL (Cranial Cruciate Ligament...the dog's version of the ACL) runs on the outside of both back legs. It connects the upper bone with the lower ones, and has a nasty habit of breaking or tearing, usually without any warning or notice.
Unfortunately, the only way to diagnose this is with a trip to the vet. Your regular vet will most likely take x-rays and palpate the limbs to look for hallmarks of the torn ligament (legs with a ruptured or damaged CCL show a 'drawer' movement..which means the bones slide side to side and front to back, whereas an intact ligament doesn't allow for that sort of movement).
If it turns out that the problem is a damaged CCL, the only way to repair it is surgically, and there are 4 types of surgery that are used and you can read more about it here: https://www.acvs.org/small-animal/cranial-cruciate-ligament-disease that will explain it to you.
Once you and your vet decide on which type of surgery is requires, recovery time is solely dependent on owner compliance and with the dog itself.
Every clinic is going to give you different guidelines, however ours went like this, and keep in mind that in between every step in the process is a trip to the vet for a check up on the surgery site:
First 2 weeks after surgery: On-leash only to go outside to potty and then back inside. Can be off-lead in the house, but stairs and jumping on couches, ect. is to be discouraged. If you are unable to limit the dog's movement in the house, confining to a crate is an option.
Weeks 3-6: Short walks on leash only. Still no free exercise off leash.
Weeks 7-12: Can be off leash while supervised. Stairs and couches are allowed. Should only walk, ect, short distances as to not stress the other three legs.
After week 12, its on a dog-by-dog basis. Obviously if the dog is allowed to do too much too quickly, they do risk re-injuring the leg, which will require surgical repair.
Of course, it SOUNDS like a long time to keep an active dog quiet...but 3 months really isn't much in the grand scheme of things. It's actually a little easier if you live somewhere that gets cold weather, as winter tends to keep people indoors more, but with determination and patience, it can be done even in warm weather with an active dog.
As for success, I've seen really, REALLY great outcomes on these dogs. You have to keep in mind that he's probably painful all the time right now because of the torn CCL, so getting it repaired will eliminate the chronic pain and get him feeling better and returning to his normal, active, pain-free life.
I hope this helps.