screaming

i have noticed before that kristaps has tendency to talk in a voice too loud. i have managed more or less to quieten him down around me. now kindergarten teachers are complaining about his shouting. it's not even shouting but more like croaking. at the end of a day he has lost his voice. he talks so loud that his throat gets all tensed. when i asked why do you shout? he answered "i don't want but it happens." i am clueless what to do because usually when it happens i am not around.