These anecdotal stories may, or may not, help…
We had one cat who began to go lame in front, really quite badly, aged around 10yo. Took him to the vet, who surmised it was probably arthritis. X-rayed, and showed me on the x-ray where you could see the arthritis (well, I couldn’t see a darn difference, but I’m rubbish at seeing stuff that experts see on xrays).
Brought the cat home with some sort of pain-killer, which the cat would NOT touch.
I then observed where and how he slept…curled up on top of his “castle,” a ratty old scratching post thing, with a dunt in the top that the cat curled up in. When he curled up (he was very long-legged), the foreleg that was underneath was always stretched back from the shoulder under his body, not under his chest or head.
I wondered if this was causing some strain in his shoulder somewhere, so I removed the castle, so he had to sleep on the bed like everyone else.
Within a couple of weeks, the cat was perfectly sound…and stayed sound until he died last year at 19yo.
So…is your collie sleeping somehow, somewhere, in an odd position that’s inadvertently straining something?
Second thought…did you have a dog chiropractor check him out? Not everyone’s cup of tea, I know, but we’ve had good success with a chiro working with a dog with a bad back in the past. That dog did the “hitch” thing behind, and also strained the base of his tail swimming. :rolleyes: - chiro’s efforts improved his comfort greatly in both cases.
I’m no expert, however, so take what I say with a grain of salt.
Good luck - sounds like a much-loved dog. 
