When your son sleeps with his head tilted back, it's usually your son's body's way of opening it's airway, which allows him to breathe better.
Your son tilting his head back when sleeping is often a sign that your son may have obstructive sleep apnea, or other sleep disorder breathing problems.
Sleeping with his head back may also indicate he has a neck muscle issue called torticollis or even less commonly, eye and vision problems.
You should take your son to the doctor or pediatrician to find the exact cause, especially if he continues sleeping with his head tilted back all the time or it suddenly started happening.
The most frequent reason for your son to sleep with his head tilted back is sleep apnea/airway obstructions, as tilting his head back, lifts his jaw and his tongue, which creates more space for air to pass, especially if his tonsils or his adenoids are large.
And tilting your head back when sleeping is also a survival mechanism for the body to get enough oxygen when breathing is difficult.
Torticollis is a stiff neck, in which the head tilts to the side, but sometimes it may present itself as a backward tilt as a result of muscle tightness or irritation.
Infections as well as swollen lymph nodes can also cause neck pain and positioning changes, which can result in your son sleeping with his head tilted back.
And if your son is experiencing GERD or acid reflux, the stomach acid coming up during the night can also cause neck spasms and cause the head to tilt back when sleeping.
In some cases it might just be the way your son prefers to sleep and is more comfortable that way, but its a good idea to consult with his pediatrician to rule out any possible serious issues.
You should consult with your sons pediatrician or in severe cases, like with choking or gasping for air he should go to the ER if he is having trouble breathing or choking.
Signs that your son should see a pediatrician are if he has any breathing issues like snoring, pauses in breathing, choking, gasping for air, mouth breathing or noisy breathing.
Or if your son has daytime fatigue, like tiredness, dark circles under his eyes or difficulty focusing.
Or if your son has any neck pain and or stiffness, like pain when he turns his head or tense neck muscles.
Or if your son has any sudden onset of bed wetting, or unusual restless during sleep.