My initial reaction is that any musco-skeletal injury can take 3 weeks to heal. I’m unsure of the timeline for this issue, but if he hurt his longissmuss dorsi muscle in anyway and was then ridden, he could have re-injured himself. I would give him a few weeks of rest (no longeing, riding, etc.) with moderated turn-out on good footing. If he’s not any better, then call the vet. This has happened to each of my horses, and two weeks off with a few days of phenylbutazone (no more than 2 grams a day, supervised by my vet) did the trick.
Also, he could be compensating for some limb soreness, which can cause secondary back soreness. Also, it never hurts to have a saddle fitter double check your saddles. My paint gelding’s saddle went from fitting him perfect to making him sore in three months. Any weight changes from summer grazing or muscle growth or loss from exercise can affect how a saddle fits. Good luck, and hopefully Brego starts to feel better soon!