My mare had lyme a few years back - she completely refused to move under saddle, was extra super grumpy (well beyond baseline normal grumpy mare) and just seemed like everything hurt. After 5-6 days of doxy she was like a new horse.
My dog and husband also have had lyme, all treated with doxy (30 days for the 4-leggeds, 2 weeks for the husband as he got acutely ill and had a bullseye rash), no recurrences for any of them. We monitor the dog yearly because she had crazy high titers.
For locking stifles, Iāve actually had the best luck using Estrone to work through it until they get strong again. The IV injectibles canāt hurt and could help. I donāt know that Iād inject the joints in this case until I tried the other things. That said, he might just need some more time off to readjust to normal.