Kids change all the time; his imagination is becoming more vivid so perhaps he's having nightmares. Having tubes put in may not sound like a humungous deal but it might be to him, even if he doesn't talk about it or show it when he's awake. Have you consulted with his ENT or pedi just in case this might be related to the tubes, in a physiological way? How is he for naps?
If it's any consolation, our LO has never been a great sleeper. He may sleep well for a month or two and then wake up every hour or every three hours. We may have to just rub his back or sit with him for a few minutes or the entire night - each night is different and we rarely know why. We speculate allergies, colds, atmospheric pressure, something he ate, something he's dreaming could be the cause but we rarely know. All we know is we have to be there for him and learn to survive on less sleep.