Get him going on a really active exercise program every day. It will tire him so he sleeps better, and get the endorphin flow going so he is less anxious.
I think he should have his own bed, I think you say he shares a bed with his brother, and perhaps you will have to start to ignore his cries when he wakes up, he knows you will go to him is the problem .,don't hang around him, it isnt good for him he will be tired it will affect his daytime performance. So let him have his own bed,make no issue out of it when he starts calling tell him once 'go back to sleep' then say no more, if he comes out send him back. be consistant, you havent been and he will continue till you are ., it will only take a few nights . good luck