Family Member Apple Watch Confusion/Wifi iphone contacts

Our son has an old iphone that he uses in the house to send imessages to his friends (so he has an apple id). We also communicated with his iphone when we need to get in touch with him when he is home and we are out. No cellular service on his phone.

We are giving him an apple watch with cellular so we can track him/call him/he can call us when he is out of the house. But, all his imessages are going to the watch now from his friends and we want to eliminate that on the watch. For now, we are the only ones we want him to communicate with.

He's a savvy kid, and if out of the house, he might want to call a friend to meet up. I'm assuming when he does this, his cell number will show to his friends....or will it come through as his "contact" name he typically uses on his iphone?

Getting super confused!