My son does exactly the same thing! He has just turned 6. It has started getting better recently thank goodness. He would scream to me from his bed and if I don't answer, he keeps on calling me. It does become habit. Try giving him a reward of some kind if he makes it through the night. Tell him if he wakes up, not to call you and if he can not call you all night long, you will give him some sort of treat or do something special. Once that helps, make it so that if he does it two or three nights in a row, he get's a treat. Etc. It is very frustrating, I know. I was getting so sleep deprived I would start crying. Good luck!