Poor you and poor baby! You sound like you guys are really having a rough time.
Has this been going on since he broke his leg, or only since the cast came off? Is it only at bedtime that he is screaming? Physical trauma can be very frightening and unbalancing for little ones. They have no way of knowing what's happening, and no context for it. Have your tried lying down with him? It may be that he realy needs to be close to someone for a time, until he regains his equalibrium. If you are still nursing, you could also try nursing him just before you put him down. Wait until he's just at the point of dropping off, and gently unlatch him. Then lay him down and rub his back for a few minutes. For other suggestions you could try www.askdrsears.com. They have a wonderful site, and address lots of sleep issues.
We have used some natural treatments in the past with our son, with good results. I really can't explain why they work, cause it doesn't seem logical, but homeopathics have helped. (We used the teething tablets, but they make one called 'Calm', it's a company called Hyland. They are totally safe, and worth a shot in my opinion. There's another one called "Bach flower remedy" which you might try as well.
Good luck to you- it's hard to see your little one in pain and upset, especially if you aren't feeling well either. it sounds like you are giving it everything you've got, just remember that when it comes to kids, *everything* is a phase!