HaHA! I'm putting even more money on the point that eaten humans become titans now (that's how titans reproduce given their asexual state). Either Eren's anger on its own was enough maintain his mind (or at least attitude towards other titans), or it required his father's magical injection.

Someone came up with the idea before about whether the Colossal Titan was a human that could spontaneously transform, so leading on from that... would Eren be able to shapeshift between human and Boxing-Titan? (I would call him Eren-Titan, but I'll wait till that's confirmed).