Hello emeshbesher,
Thanks for using the Apple Support Communities. I know that iMessage is a great way to keep in touch with friends and family, so I'll be happy to provide some resources to allow you to reach your son again.
Since you've already done a lot of great troubleshooting, such as turning iMessage off and on again, and removing the contact, it sounds like the phone number may not be pairing to the iMessage account as we'd expect. Check out the section of this article labeled "If contacts see your email address instead of your phone number when you send messages" and make sure that each of you have the preferred contact option chosen:
If you can‘t send or receive messages on your iPhone, iPad, or iPod touch
If contacts see your email address instead of your phone number when you send messages
Your iMessage settings may be set to receive iMessages and start conversations from your Apple ID instead of your phone number. Check your settings to see if your phone number is set to send and receive messages:

- Go to Settings > Messages.
- Tap Send & Receive.
- Make sure that your phone number is listed and selected under You Can Be Reached By iMessage At and under Start New Conversations From.
If you don't see your phone number listed, you can add it. Follow these steps:
- On all of your iOS devices, go to Settings > Messages > Send & Receive.
- Tap your Apple ID, then tap Sign Out.
- On your iPhone, go back to Settings > Messages > Send & Receive.
- Tap Use your Apple ID for iMessage, then sign in with your Apple ID.
- Make sure that your phone number and Apple ID are listed under You Can Be Reached By iMessage At.
- On your other iOS devices, go back to Settings > Messages > Send & Receive.
- Tap Use your Apple ID for iMessage, then sign in with your Apple ID.
Once you've both set this up, attempt to send an iMessage to his phone number once more. If that doesn't resolve it, are you able to send an iMessage directly to the Apple ID used on his iPhone?
Cheers!