A.A.
The tongue thrust, when your son pushes out the food, is a sign that his body is not ready for solid food. It's nature's way of protecting him against foreign things in his mouth. The AAP even recommends that babies not start solid food until at least six months. Your doctor is absolutely wrong in telling you to feed him so early! The Dr. Sears link someone gave you below has great information. The signs that your baby is ready to start solid food are the loss of the tongue thrust, able to sit on his own without assistance, have an interest in what you are eating, and have the hand control to pick up small objects (the pincer grasp). If your baby doesn't have these signs it means his gut is still not fully developed and eating solid foods makes him open to getting food allergies, not to mention indigestion. Waiting until six months or longer will not hurt him at all, it will actually be much better for his health. Solid foods are only supposed to be for play and practice the first year. As long as your son is gaining weight and is happy formula or breast milk is providing all the nutrients he needs.
By the way, I live in Lexington!