Police said teachers had alerted the fire station after the boy began crying and climbed out of a classroom window.
It was reportedly his first day at the special needs school. Efforts by the teachers to persuade the pupil to come back inside had failed, BCC added.
But a remark by his mother about his passion for comic superheroes prompted fireman Somchai Yoosabai to rush back to the station, where he kept a Spider-Man costume in his locker.
The sight of Mr Somchai dressed as Spider-Man and holding a glass of juice for him, brought a big smile to the boy´s face, and he promptly threw himself into the arms of his "superhero", police said.
Somchai normally uses the costume to liven up fire drills in schools.
Spiderman swinging its way to Nepali audience’s heart