My ds use to eat vegetables, farex, fruit and a...

My ds use to eat vegetables, farex, fruit and almost anything he could get his hands on. Now he wont eat anything except pureed fruit. If I try to give him anything else he screams. He has 2 teeth and I am wondering if teething has something to do with it. I am worried that he isn't getting enough food because he has started waking up a few times in the middle of the night (he usually sleeps from 9pm to 6am). He is also underweight according to the doctor, because he only put on 400g in 2 months. and is 1.5kg under the average 6 month weight. The doctor says there is nothing medical wrong with him. Am i doing something wrong?? I feel like a bad mother. Please help??