Setting Up iPad for My Young Child

Set up your son with his own child Apple ID, then set up Family Sharing on the Apple ID with the purchases. Family Sharing allows up to six family members to share apps, etc., but not In-App purchases.

Apple ID for children under 13

https://support.apple.com/en-us/HT201084

Setting up Family Sharing

https://support.apple.com/en-us/HT201060

Unfortunately almost all in-app purchases (including subscriptions) are non-shareable for non-Apple apps and services. Most Apple ones are. You can check in the App Store - there’s a Family Sharing icon under the app description if it is. This article has more details:

https://support.apple.com/en-us/HT203046

As I understand it:

• Almost all apps, free and paid, are shareable; game progress might be lost, depending on the app;
• Most in-app purchases of “consumables” will not transfer;
• Subscriptions can be stopped under your wife’s ID and then purchased afresh under his ID (she’ll still pay for them).

What types of content can I share with my family using purchase sharing?

https://support.apple.com/en-us/HT203046

This will allow you to set up his own Apple ID and share content, and still have parental control of the device, Set up Family Sharing - Apple Support