I really hated Eric, and not just in the sense of him being a rival. I hated him also for how his arrival suddenly made Ann and the rest of the family stupid and completely disloyal to Max. And on a completely meta-level, I hated that the character existed at all. It's not that I'm completely opposed to male rivals in games. But I need the option where I can either avoid them completely or at least keep them away from my potential love interests if I do everything right. That wasn't factored at all into this game. With Eric, it was all about minimizing damage by making choices that were oftentimes counter-intuitive. Except I don't think DS imagined that anyone would want to minimize Eric at all. I think he was meant to be the "mentor" that let you have his sloppy seconds, and you were supposed to be best friends with him. And as you mentioned, there's the ridiculous part about military school, but there's also Ann completely disbelieving her son to an unbelievable level.Well, that's kind of the point of Eric... To get in Max's way and provide the (ludicrous) Bad End of Military School for a selfish pervert who's at least 18, was kicked out of school and doesn't have to go back.
Unfortunately, due to the ham-fisted implementation of Eric and how he affects the household, anything approaching a plot went out the window with him. Patrons were really pissed off.
There was definitely foundations with a lot of potential (including some minor "plot" advancement), but the money dictated it go out the window and focus exclusively on fucking all the women, to the point that even Kate changed from being a lesbian, to bi-sexual with predominantly lesbian feelings... except for Max's dick, of course.
And Eric just kind of suddenly dropped into the game an update or two in before almost immediately taking over. He would have worked a lot better as Ann's already existing boyfriend prior to the move who suddenly developed an interest in Max's sisters as well.