Sharing Apple ID on another device

It will continue until he has his own separate Apple ID. (His phone is your phone … and vice-versa)

If you desire, Apple still provides robust parental controls via Family Sharing.

Setting up separate AppleIDs is beneficial at every level. (usability, manageability, and security)

See details here:

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

Apple IDs are personal; they shouldn’t be shared any more than you would share a toothbrush. If you share an Apple ID with someone else many bad things will happen, not just messages. He should have his own Apple ID, then you and he can join Family Sharing→What is Family Sharing? - Apple Support. That way you can share what you want to share and not share what you don’t want to share.

It is OK to share an Apple ID among devices used by one person, such as an iPhone, an iPad and a Mac; in fact, it is a very useful thing to do. But not between devices used by different people.

Here is Apple’s FAQ: https://appleid.apple.com/faq/#!&page=faq. See the 4th bullet item.