My 18 month old son won't say any clear words.?

My son is 18-19 months old and does not say any clear words. He does use certain "words" for certain things-for example, he says daa for dog, dat for cat, and dit for dirt. He can do some animal sounds-he growls for tiger and moos for cow. He is very sharp in every area and knows and does anything I say. He points to many things in books-where is the cat, where is the tree ect.- and puts together wooden puzzles. My question is, are there any moms out there whose son is the same way? What advice to you have? If your son was this way and now speaks, what did you do to help him? Should I worry or see a speech therapist yet? I am a stay at home mom and I know what he wants. I have tried getting him to speak by not giving him what he wants, but, it never works, he just cries or forgets about it. Any advice would be great.