Thank you for your query,
Nothing to worry as there are no other sypmtoms are associated with.
As he tired and not taken his nap, he was sleepy and could have slept for this long duration as the age group children play excessively at school and will slept with tiredness
When he wakes up look for any concerning symptom of illness and if it is like that consult your Pediatrician
Hope I have answered your querry.