Hey Boxer,
How much weight do you still need to loose?
You look great my friend...if that's you in the picture. But if you meant staying fit while in cast...then I don't think your arms aren't able to maintain some light exercises...legs as well.
Stay on a balance healthy diet for fast recovery...
I can recommend a nutritional supplement that is use by Olympian Athletes in Europe and Australia (they beat the competition without illegal drugs)...even Farrah Faucet, Magic Johnson has been taking this as their secret supplement to stay fit.
Good luck...