Hi ,my Son Is 4 Nd Half Year Old .He

Hi ,my son is 4 nd half year old .He is very talkative in home with parents and few relatives he also talk little bit with classmate.but he never talk with his teacher not a single word and never talk with outsider in public.if we force to talk days hello or something to anybody he can't speak.some times he try to talk but his volume is very low.i just want to know why?Is there any problem with my son.if yes than what happens next to him?