My pedi told me to stop breastfeeding

My son recently turned one and since birth he's been a small boy. He was born a bit early as well. He's always followed his lenght curve, but has been low on the weight. At our one year check up appointment my pedi told me I have to stop breastfeeding (have been exclusively breastfed since birth) him since that's what stopping him from gaining weight properly (according to her) and I'm devastated. I had planned to breastfeed until he wanted to stop, or until he was about two. I really need some encouraging words right now 😢