Will try to keep this short.
DC's dad and I are not together but we are amicable, we chat he comes to see dc's activities etc.
Yesterday we were swimming and he and his parents came to see dc. All fine.
Ds has autism, dyspraxia, sensory processing disorder and hypermobility. For those who don't know, hypermobility often means poor posture because of low muscle tone. Ds is often slumped and he looks pale and his skin looks "loose" for want of a better description. Ex started discussing with his parents how ds was "filling out" looked fat, was eating too much, didn't get enough exercise.
Ds is NOT fat, he is bang on his BMI but yes does look unfit when he walks and moves BECAUSE of his hypermobility and dyspraxia!! 
This convo went on for ages, luckily ds didn't hear. In the end I stepped in and told ex to basically STFU (didn't swear) and that he didn't know what he was talking about and should bloody well read up on his kids condition.
All I could think was how hurt and unconfident ds would feel if he heard that convo (he's 11). Ex said it was harmless to discuss that kind of thing with his parents, because they're his parents. They are lovely people but it's a running joke how large one of the cousins used to be as a child and I don't want ds to be spoken about like that.
So AIBU?