Trouble gaining weight

We just had our 15 month appt today with the pediatrician. My son has not gained weight since his 12 month appt. He eats constantly, always wanting bread or yogurts or bananas. Since taking him off formula and weaning from the bottle, he completely refuses milk of any kind. We've tried regular, soy, almond, chocolate, he hates it all! Our pediatrician said to try pediasure to help gain weight. I'm worried he's going to hate it since it's similar to milk, but maybe it's close enough to formula that he will like it? Should I try putting him back on toddler formula? Any foods I can try to help put weight on quickly?