How do you keep your 6 year old from waking up in the middle of the night?

My son has been a difficult sleeper since he was an infant. He sort of goes through cycles of waking up in the middle of the night, then sleeping well. We have tried exhausting him during the day, melatonin, and nothing seems to help. He will get up, play, watch tv, eat, and is now waking his sister up. I am exhausted. We have three kids and he has started Kindergarten. There is NO WAY he is not exhausted. We are to the point we are locking him in his room but last night he awoke screaming his sisters name. My husband got up and fussed at him. I am going on about 10 hours of sleep for the past two days. HELP!!!! I am about to try Benedryl. The Dr suggested the melatonin and we took a break from it over summer because you can build an immunity to it but it helps him fall asleep but not STAY asleep.