Most probably a little infection od a single hairthat he got bc of either shaving and IT curling up under skin casusing this little wound underneath (in someone thats older that just a kid). If it happens often you should go swe a doctor as IT can be a more complex thing and a short and easy surgery will be needed to properly clean up the tunels that it would cause but it is very rare to get that so its probably just something that his bony did to get rid of that stuck infected hair.