But he doesn't know about Max's feelings until after he is already in a sexual relationship with his mother...
There is no relationship other than Mother Son...
By your definition, His father if he was in this game would also make this an NTR game if he slept with his mother...
I prefer the Urban Dictionary definition...
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Max is never her partner in that description so the only one being Cucked is Eric and only after Eric is gone with the exception of a HJ you can get when massaging her while he is still around.
If Eric had appeared AFTER Max had the sexual relationship with his family only then it would fit the situation you are looking for.