What's a polite way to tell my parents to not comment on my son's weight?

I love my parents so much but they get butthurt easily and when they're upset they stop talking to us kids for a while. So I try not to upset them. My son's weight is a sore topic. I have 5 kids all boys and I'll be honest, I didn't not want girls. I didn't want to deal with the drama and mental health issues of having girls. Before you call me ignorant I already know and learned quickly how ignorant I was when my son had mental health problems I thought only girls had. One being an eating disorder. And it started quite young. He was only 10 and would hide that he was eating or throw it up. He got hospitalized for it a couple years ago. The eating disorder caused some permanent damage and health issues we are working through. When he was going through an ED my mom commented constantly said he was skinny. Then when he came back from rehab they said he had gotten too fat which isn't great to hear after being in rehabilitation for an eating disorder. Then he started antidepressants and got more chubby and they really started with the fat jokes. At the moment we are working on intuitive eating and staying healthy. I don't push if he says he's not hungry as long as I've seen him eat something that day. But my parents keep comparing him to how he used to look and how he's almost 100 pounds heavier. I wish they would just stop and leave him be. Me and his dad even if we aren't together are really trying to work with him here.