Deactivate imessage

We got our 8 year old son a Gabb Watch. He can only send/receive text messages to/from numbers we pre program in the app. His “phone number” on his watch just have previously belonged to someone who used iMessage so when I try and text him (from my apple) it try’s to use iMessage and he doesn’t receive my messages. I tried to deactivate his phone number for iMessage but it sends him a text code but since whatever the number is texting isn’t on his “approved” numbers he doesn’t receive the code. So I’m not sure what to do. When I turn off iMessage on my phone he receives my texts fine. But that isn’t a long term solution. Any ideas on how to deactivate my sons phone number from iMessage without the code? Or at least can someone give me the phone number apple texts from so I can add it to his approved contacts temporarily?