Time,
Just a suggestion from another lab lover.
Most times with labs they run and run till something like this happens. It happened with my Sadie. One thing you can try that won't hurt Bo and just might get some muscle tone back, help with the hip displacement and arthritis. It's called Glucosamine chondroitin complex with msm. If you can't do the vet thing again you can buy this at Walmart and thru it is expense. It gave more time with Sadie and do help her a lot.
Will keep praying for Bo and you.