Sleeping problem and melatonin. Sleep.

My child is two years old. He is having sleeping problems, wakes up after 1 hour of sleep at night and scream, then sleeps only after 5 in the morning.even then wakesup hourly. Consulted a pediatrician and melatonin is given..this is the second day of the medicine and he is having the same problem..i am 8 months pregnant and he needs me whole night..somebody please tell me how long it will take to make my child sleep whole night..i feels depressed and stressed..he is very difficult to console and scream for long time..no other problems are found..digestion is ok. But have reluctance to eat. No cold. He never slept during night from birth..is there any solution to this.is this a serious disease or something..i am feeling extreme tension since doctor said he may need to take MRI of him..can melatonin give long hours of sleep.?