I think your falling into a trap here, that "normal" people don't bang their mothers, and yer trying to justify why it would make sense to do so.
He's also falling in the trap where the son feel a guilt so strong that he want to die, before feeling the total opposite feeling once totally separated from her, while the mother follow more or less the same path. It would needs years before he overcome the guilt, and years again before he accept that he now feel a void inside him because he's no longer with his mother.
He is putting the realism from the point of view of a son who's sexually attracted by his mother, but feel guilty from start to stop. It's biased from the start, because the premise prevent the act to happen. But in real life, incest do not works like that. Unless it's rape and/or mental manipulation, the guilt trip start before they are doing it, while they'll finally do it because they already decided to stop caring about this issue.
Whatever if it happen because of pure lust or because of love, consensual incest is always the outburst of feelings that grown for years, and therefore that already overcame all guilt. If the last thought of the protagonists should be summarized, it would look like, "fuck morality, I'll do it". And it's exactly what happen in real life, what anyone who is/were engaged in a consensual incestuous relationship, say about this first time. They were there, doing something totally unrelated, and the instant after this they were doing it for the first time.
What matter for the son/mom incest story to be effective real life-like is what he don't talk about, the premise. And the one that will feel the more guilt will always be the oldest one ; because it's the one that will feel responsible for the future of the other. Real consensual incest looks way more like
this, than like he described it.
As for none consensual incest, it's just a rape, and therefore here again it don't works like he described it. The rapist do not feel guilty. Why should he, from is point of view he did nothing wrong. As for the victim, it's also no guilt that is felt, it's hate. It's possible that, after a long time, the victim decide to forgive, but she'll never forget and will never stay too near to his/her rapist, and even less alone with him/her. What once again make his scenario nothing near to the reality.
