My son is super skinny...help!

My son is 7 months. He was born at 37 weeks but was 9lbs 4oz. We had some weight gain and jaundice issues at the beginning of our breast Feeding journey but then he ate like a champ and was growing steady til about 2 months ago. He is gaining weight now but super slow...like an oz every 2 weeks. My pedi recommended supplementing with formula, but he won't take a bottle and when I try to mix it with something, he spits it out. He only wants Mama's milk or his baby food. He eats baby food very well. Anyone 3lse had just skinny babies? He's 7 months now and weighs 14lbs 9oz.