I just don't know what to do!
He enjoys time with his dad, goes every other weekend and one night in the week.
But every time he goes to his dad for a week he comes back with severe constipation (says he didn't poo at all while there) and a urine infection.
I messaged dad after last time saying it had happened yet again and could he remind DS to go/make sure he is comfortabek / encourage him to drink lots. But just had an angry and defensive reply saying Ds goes twice a day at his and he will take photos in future 
Just don't know what I do next? Tell GP (but how to discuss with Ds (6) there?). Try to raise again with exH? (But he won't listen)?