mr. serious and ms. playful

With our two children, our daughter is always the jolly one, "always on the go" type. Our son on the other hand is always mellow, silent and serious. As you can see in the picture, he's in the deep thought while she's in the playful mood. Sometimes I worry about our son because his development is a little bit late compared to our daughter. He is turning two in 4 days but his words are not complete. He has this installment type of saying the words. Say for example the word "water" he only say "wa" he seldom complete the word. Our daughter started talking as in talk in sentences when she was 18 months old. Need some suggestions, tips or any advise you can give.. Thanks a lot!