12 year old dating
Industrialization moved that age back, but our bodies don't know that.
I've recently noticed myself thinking of college graduates as emotionally immature.
That ties it to specific, relevant milestones rather than an arbitrary number (plus, it reduces your role as either gatekeeper or enabler).
We had a similar question a while back, where I outlined my own personal policy and my values backing that policy.
In addition, the majority of the dating tweens had met one another's parents.
Tweens tend to pick their boyfriend or girlfriend based on the person's looks, clothes and/or social status.
When developing your own policy, you should consider your own values and the results you want to achieve. It isn't a choice between forbidding any relationship at all or permitting any relationship at all.
In other words, instead of forbidding the date, require adding more people to the event, put restrictions on being paired off or alone, and encourage him to develop a wide variety of relationships.
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This question seems really vague but it doesn't seem like the scenario is going too badly with your current approach.
For one, tween relationships are usually not kept secret.