did I read it right that previously he used to barge into the bathroom,?
To what end? To try to catch you out/check you were not having a poo?
This IS abuse. Even if everything else is great, this is the literal 'how much poo in a cup of coffee makes it OK' abuse
even now, even when the light has been shone onto it and others agree with you, he still doesnt believe it really. He has said 'OK bloody poo then' but then sent you a link to another man with warped ideas to justify why he is correct.
Im guessing it comes from his childhood - its that deeply ingrained. You say his mother was controlled in the same way - he has it in his mind from there, for whatever reasons. But that doesnt help you now, does it. And frankly, even if you get over the pooing thing yourself, I fear he will be so angry about it that the abuse will pop up elsewhere.
He probably doesnt even know why he does it, its so deep. Which is why he wont be able to explain why he will get so angry when you ignore him. Its about defensive reactions, pure survival instinct for him, as its wrapped up in his own abuse. Its a desire to control to make him feel safe. So when you recover and are able to function digestively, its going to rock his security to the core, and thats not a good place to be when you have someone dealing with extreme PTSD from abuse. So this seems a silly and joking matter but I really worry that it could get very nasty.
Controlling someones bodily functions is about as abusive and controlling as can be. It doesnt matter WHY, what matters is that he is doing it, and has trained you, via shame and fear, to comply with his demands. And to realise that when you fail to continue, or take steps to regain control of your body, that the abuse is likely to escalate in other parts of your life as he feels his control and safety slipping. Again, he may not be able to verbalise it, he may not even be able to say that this is the reason, he may not do it deliberately, but you will be left with dealing with him in a raw state, with all his insecurities rising to the surface and he will go into complete PTSD survival mode.