This is a very common concern of parents. My sister didn't talk until she was four. Then she just started talking like a normal four year old one day. She never stopped, unfortunately.
I know it can be frustrating, but if your little one understands everything you say, then I wouldn't worry about it. If there is no speech problem, as indicated by your doctor, then you will wake up one day and your child will be talking.
I second the reading advice. Reading to children is the best thing you can do to encourage speech. Interactive reading (i.e., asking your child to describe the pictures) will go a long way in encouraging a reluctant child to speak.
Good luck.