Hopefully this isn't a cliche in itself, but what about giving him the power to absorb life energy through his roots? Hated, youthful humans, more hated the younger they are, become weak and lethargic after only a few minutes within a certain radius. Spend too long within his sphere of influence and watch the years begin to accumulate on your own body. So he engages any visitors in conversation, biting back the bitterness and smiling stiffly until his victims realize what they have lost by coming too close for too long. And then, when he sees the realization dawn upon their newly wrinkled faces, he cackles with glee.