18 month old not talking?

[Joe]

My son is 18 months old and does not speak. He points at things and says dis. The pediatrician says he is way behind and at a nine month old level. She wants my wife and I to consider having the Department of Health come in and evaluate him. However, I've spoken to many parents who say to just wait and he'll talk when he's ready. I heard some kids don't speak until they're two or even older. I also have a 3 year old daughter, and my wife and I kind of compare my son to her, and I know we shouldn't since all kids develop at their own pace, but my daughter was talking at a year old. My son seems to understand things when we talk to him, he looks at us when we call his name, he even will bring my wife and I books and likes to be read to, but nothing comes out of his mouth. I do notice that when we speak to him, he looks at our mouth like he's studying what we're saying, but he doesn't even attempt to try to repeat anything we say. I'm for waiting until he's at least two to see if he speaks. My wife on the other hand is concerned and is taking the position of the pediatrician. Can anyone help me with this or give me some ideas on what I should do and if this is normal. Thanks.