[He's basically always hiding something from his parents, though in this particular instance nothing he's hiding is actually relevant to the current situation. Especially since honesty has the possibility of hurting more:]

All I told him was that you were here and I hated you.

[Names were left out of it, because he didn't see the point in details. After Anakin so vehemently rejected his future, Ren lost interest in him.]

Not that I would have been opposed to telling him your identity, had he asked. [So feel free to continue being angry at him.]