Child wont eat

Ok so this may be long. I have a 3 year old boy. His whole life he has refused to eat. We have had him to his regular doctor, a gi specailist, a dietician. They have done blood work, stool samples, dye tests of his stomach and throat and cant find anything but acid reflux. Even as a baby he would not drink the normal amount of formula. He was almost 4 months old before he would drink 4 ounces. He is not gaining weight. He only weighs 27 pounds... His height is good, he is very active, very happy, and otherwise healty. We have made some progress with the foods he eats but not alot. Today he ate one time... would not wat anymore and yoi canot force someone to eat! Has anyone had any experince with this? Im gonna make him another appointment. I want them to check him for autsim. I see alot of children that are autistic have this problem... he talks but its not great. He cant say his abc or count... and I have tried and tried... any suggestions or similar stories..