When someone has already made a decision, there still might be a way to get them to reconsider.

Objective?

Get them to reconsider? Get them to rescind that decision?

How do you get someone to reconsider a recent decision?

Find out who he made the decision with?

Ask them what it will take to get them to reconsider?

Give them a reason to reconsider? (Better features, benefits, values, experiences, price).

Get them to doubt the soundness of the decision?