I feel like Caleb underestimates the love a child can have for their parent, for the sole reason that they're their parent, especially at that age. Kousei loving his mom doesn't HAVE to be manipulation on her part, as its not unheard of for victims of abuse to internalize it themselves, creating excuses for their abusers, rather than their abuser create excuses and the victim believe it (though that does happen as well).