Perhaps contact someone in kiruv and ask for advice.
Sometimes people are told to keep contact, for the relationship itself may keep the other interested and connected to Yiddishkeit in the future, as long as it doesn’t affect you negatively. It varies on the individuals involved.
Perhaps the kiruv organization has someone else trained and qualified to debate him in s& p, since he’s “mostly interested in Science and Philosophy.”
Wishing you both much hatzlacha.