How do i (grandfather) transfer apps new apple user has used on my device to his ipad?

Assuming you have an Apple device, you can do this through Family Sharing: Set up Family Sharing - Apple Support.

Then, open Settings and tap Family > Purchase Sharing > your name > turn Share Purchases on.

He will then be able to download the apps you have on your device by opening App Store and tapping his profile picture at the top right > Purchased > your name.

If you are not using an Apple device, sharing apps is not possible.