Hi. Our son lost his ipad. We tried to use Find my app but it does not show anything. Can you help please?

If you use an internet browser and log into www.icloud.com/find with his Apple ID and his device doesn't appear there, see the information in If Find My iPhone or Find My iPad isn't enabled on your missing device - Apple Support including the following

If your missing device doesn't appear in the Find My app or at iCloud.com/find, you didn't turn on Find My. There's no other Apple service that can find, track, or flag your device for you. But there are steps that you can take to help protect your data.