I'm not sure if your son is waking up because of the LQTS, but here are the triggers I've read about:
Increased physical activity (especially swimming),
Emotional distress (crying, anger, or anxiety),
Sudden and loud noises.
However, some patients experience symptoms at rest or during sleep.
Certain medications may prolong the QT interval and should be avoided by patients with LQTS or a family history of LQTS symptomsI've heard the triggers are mostly
Is your son taking medication?
Caffeine could also cause him to wake up and be agitated. I would imagine that a lot of sugar like in a soda could also cause this.
It sounds like your son could be having night terrors. I think they are pretty common in young children. Is your son on Beta Blockers for the LQTS? I believe that some people have nightmares while on this medication. Certain triggers depend on the type of LQTS your family has. Our family has LQT2 and our triggers are more often from sudden noises or frights and extreme stress and sometimes during exercise or sleep. As long as people take their medication and avoid certain medications on the Long QT Drug List episodes are usually under control. Has the rest of your family tested yet?