Welcome to healthcare Magic!
10years old child is grown up enough to surely say he has not bumped into something, you should rely him.
may be due to some insect bite that the child might have missed, in this case may be he has noticed sudden pain and burning at site and the central lesion appeared suddenly.
Another possibility is of a boil in which case the onset is typically slow with pain first and then the swelling and then the white central lesion.
In any case the child must be reviewed by a local physician who will finalize the diagnosis and will know the appropriate treatment strategy for treatment to be followed.
I hope the advise would be informative and useful for you.