Daisy. Do you know for sure the leg is broken, (did you see how it happened) or is there a chance that he has only lost the use of it for some reason?
Are you sure it is a male bird?
How many months old is it?
How many other birds was he in with? How long had he been with them?
It's great you got it separated and in a smaller area. Make sure the box is lined with lots of padding like thick paper towels or something and be sure to keep him warm. Also keep him calm and quiet.
Do you already know of a competent avian vet?
Let me know about these questions and we will figure out where to go from there.
Thanks, Patricia