Can you post photos of him?
Where did he come from? You mention he is a rescue - what was that like?
I'm very sorry, but if at 4 months he is unable to walk/is too heavy(?) then the quality of his life doesn't sound very good.
Do you have photos of him?
Are his legs deformed?
How old is he?
A sling may be helpful in giving him support. You would still need to monitor him in a sling, he could flip out or may not be agreeable.
Red Rangers are meat birds, but they are also bred to be a bit more "natural" and...