Sleeping at night

I am really praying someone has been in my boat and can offer any advice....my three and a half year old,son has all of a sudden developed a sleep anxiety at night- i initially thought it was that I had taken his bed rail of his bed to give to a friend who needed it ...he had been in a big boy bed for two years so thought it was ok e then began having night terrors....I bought a new bed rail he slept fine for three nights then it as a thunderstorm.....he was up the entire night and for the past five nights he began screaming bloody murder when I would try to turn the lights off and leave his room...we gave additional night lots and he already had a sond machine I also gave him a flashlight for him to hold....when he would finally fall asleep he would proceed to wake up during the night....the dr suggested melatonin but I feel like it is a short term fix or has anyone else needed to use it and had the child get back into his/her sleep pattern? We have never had any difficultly until now and I am just at such a loss and have such anxiety myself now! I worry constantly about the next storm and how to break him of this awful cycle :-(