I am not a doctor or therapist, but, fwiw, constipation in a kid can be a control issue. In a home where a kid is perhaps feeling oppressed or controlled, withholding bowel movements can be something s/he can independently control - and that can lead to constipation.
It's a way to assert or feel some personal control without "acting out" - and to me, it matches how I see Gannon. He doesn't seem like the type of kid who would openly fight authority being mean to him. Perhaps he quietly rebels against a confusing, chaotic, unpleasant home situation by hunkering down and withholding bowel movements - not that I think he consciously does this, but rather that it could be his reaction without thinking. It's almost a way of disappearing, to deny one's self a body function.
That is what I've been thinking from the start when I read he might have constipation, but didn't mention it as I'm not an expert, just someone who has made observations over the years.
There might be medications that lead to constipation too, but idk about them or what or if he was taking any.
I don't claim this is a fact but simply offer it as an idea. I have no links, only my own opinion.
jmopinion