I agree with @Forgunia. Even if he respects his mother's choice, won't a loving son normally speak his mind? She's married, her son is also living under his roof. To let her relationship going is irresponsible. I think he's fucked up in the head on this matter. It's so sad given he's pretty rational/responsible in other matters. But I suppose people love reading this kind of train wreck. Not me tho, I only read this far hoping the horse-dicked brat and his easily swayed lover get what they deserved; get thrown out the house.
I checked until part 5, and sadly that kind of ending (or any other satisfactory ending) is still not happening. I don't think I can keep on reading. Being stringed along is not fun.