Infant reflux & weight gain

So my son's pediatrician is telling me because he is gaining the weight he needs to gain that he doesn't have reflux. Everytime I give my son Similac sensitive he's spitting up and then is having painful gas. I cry watching him squirm and squeeze. I had reflux my last trimester of pregnancy and I know that painful acid feeling. I asked her to switch the formula but she refused to give me a prescription for Similac alimentum. What formula is best for this situation? I am so sad because I looked up videos of reflux in infants and my son is doing the same thing they're doing. I am changing his doctor.