Late, I know, but IBS can be caused by either a infection or a stressful life event. I've met someone who had something similar (though I think it was Fecal Incontinence) and his mother used to slam his diaper filled with poop in his face and was really abusive overall.
I mostly called him a fag because of the way he acted when he was 6-7 (though nowadays he's getting better). So maybe Ian really did get abused by someone. I'd bet its on his father considering he fired his coach because he reminded him of his dad and because (in my personal opinion) he's scared shitless of Sam Hyde.
So, maybe maybe, with no proofs whatsoever and just as a personal opinion, Ian was abused by his dad on the bathroom? And the reason he stays with Anisa is because she's the only one that accepts him because of that?
Though I just think that whatever he has is just his own doing, not the fruit of an abuse