How do I block all access to youtube on my son's iPad?

Hello. Have you set up parental controls/ScreenTime on your son’s iPad yet? If not the following support article should help you set this up, where you can prevent apps being installed and add YouTube as a blocked website.

https://support.apple.com/en-us/HT201304

I would recommend that you delete YouTube from the iPad then restrict installing apps.

1. Go to Settings and tap Screen Time. 
2. Tap Content & Privacy Restrictions. If asked, enter your passcode.
3. Tap iTunes & App Store Purchases.
4. Choose a setting and set to Don't Allow.

You can also change your password settings for additional purchases from the iTunes & App Store or Book Store. Follow steps 1-3, then choose Always Require or Don't Require.

Howard