DS is 18 months old and he has still not said a complete word ... He says only half words (or only the first two letters of the word). For eg. Ball is "ba", cat is "ca", book is "boo". The only words he does say is "dada", "mama", "nana", "papa", "baba". I read somewhere that he should be saying 20 complete words by now! He says about 50 half words ... Should I be worried?