Again, I'm not a vet, I just helped a few maintain their status of living with my Cocker Spaniel.
Well I can tell you this shit stumped several vets regarding my Cocker. I'd also look into breed specific problems. Cockers are known for skin issues but also for a bunch of other problems. So bear in mind this might not be just one issue (which was my case and why it was so damn hard to fix).
The vet will wanna do an allergy test at some point if it's not something obvious.
That's why I bring up the prednisone as a last resort. It's not something you wanna have to give the dog daily forever BUT it was the ONLY thing that gave that poor Cocker respite for the last five years of his life. It was a TINY dose that did it, but still. Just something to bring up with the vet if all else fails.
Before he was on it for life though he'd get a round of it and it'd clear him up for a while. That may be your case, it's like a silver bullet for a lot of problems it can just be hard on the body. ANY steroid like that can be bad though and there's a reason why prednisone can be especially bad, so for sure talk to a vet first.
I CAN tell you this: if you don't stop the chewing, even if it resolves, the problem won't. You're gonna NEED an e-collar (elizabethan collar, "the cone" or whatever you wanna call it). It becomes a habit for 'em and it sounds like it's been going on long enough for that to be an issue. My setter will get an itchy ass from time to time, will just quietly and calmly suck on his asshole for hours like it's the tastiest lollipop in the world if you let him. Problem goes away and then I have to break him of the habit for a few days again. Mine doesn't mind the collar at all, now nail trimming is different...
There are a few sprays that help with chewing but I forget which ones work, it's been a while. It had something in it though, wasn't homepathic or whatever. You do wanna be careful with topicals, talk to a vet, because they'll just lick it off. You don't want them licking something they shouldn't.
Poor pup. BTDT, it sucks.