my son doesn't say many words he is 19 mths old..he understand everything, recognize who is who and what is what..but doesn't repeat the words I say..when he was 12 to 15 mth he would mispronounce few words like baloon n ball now he doesn't do that..he does mumble ma ma n papa when he likes..n few jiberish words, ...what is the reason..is this normal for this age..how can I make him talk