Are you comfortable with petitioning saints to help you? This sounds as though it might be a task that Saint Anthony would be inclined to help with. He's known for helping lost children, but I think that helping a child's mother return to the child would be something he'd go for.
If you're comfortable, tomorrow (or any Tuesday) find a candle (his color is brown, but brown candles might be hard to find, white would be fine), and use his prayer. Talk to him and connect, offer him a small shot glass of water for helping you. Keep praying/talking to him every Tuesday, lighting his candle. If you intend to make an offering to him, ensure that you follow through.
If nothing else, it may help open lines of communication for your son and his mother.