Talk to his piano teacher and tell her of your child's fear. She should be able to offer some advice and help talk to your child to encourage and build his confidence.
Continue to support your child. Let her know that being nervous is a normal reaction and that everyone, even grown-ups, feel it to a certain extent when they have to do a recital.
Listen to what he has to share with you and address his concerns in a kind and respectful way.
Help her choose the music for the recital or she can pick what she likes to play.
Encourage him to practice daily. Daily practice can help him to gain confidence as he will get more comfortable with his piece.
Help her to pick out the clothes she will wear for the recital ahead of time. That way, she has one less thing to worry about on the day of her recital.
Remind him to relax, not to worry and just enjoy playing his favorite piece.
Reward her after the recital whether she makes a mistake or not. Let her know that you are proud of her either way.