Advice needed!

Okay, so long story short, 6 weeks ago, I brought my 8 year old bun to the vet for a routine nail trim and get handed back a traumatised rabbit who has a broken leg 😭
Anyway, the cast is now off and my poor baby has complete muscle wastage in the leg.
So does anyone have advice on what I can do to help Flopsie build back the muscle in his leg? Any advice would be much appreciated!
TIA