Please stop deamonizing his auntie. She was mother to him, too, regardless of how much it hurt you (and him). Yes, you were victimized but your child should not carry the burden of that. You are dividing his loyalty and he probably feels bad because he loves her, too, and you're making him blame her for your hurt feelings... it's horribly confusing to him.
I don't understand your situation, honestly, and I'm not saying that to be judgemental. I just don't understand. I would get him counseling. Reconsile with Auntie and be open to both of you loving him and spending time with him. Why did she return him? He must have felt like he wasn't good enough for either one of you, even if you know that's not true. You can tell him it hurt you when she took him, but it's years after he suffered the pain of losing you... then losing her and he's already processed those events and assigned feelings (likely abandonment) to them.
Seek therapy, individual and family. Good luck.