iPhone 6s used for child, iMessages suddenly stopped working

We did a total reset to factory settings on an old iPhone 6s for my son to use. There’s no SIM or phone number but I keep all the kids on the family plan (I have 4). They all have older phones from me and my husband that we did factory reset, they each have their own Apple ID, and we have “allow” for accounts, messages, and FaceTime.

Yesterday he was messaging all of us and it came up as his Apple ID and name and everything, but today suddenly under messages, it is greyed out to sign in (although he is) and I keep getting activation error to set up FaceTime. Yesterday I also had to restore network settings to make his work, and his is the only one of the 4 old phones that won’t message now. I’ve googled everything and tried all the suggestions, but his phone doesn’t “toggle” on and off- only mine will do it in parental controls. I’ve toggled them on and off, restarted the phone, but he can’t even sign out of the phone because it’s grayed out, to sign back in. Why is his not working? Everyone else in the house has their Apple ID and it’s working fine with no phone number or sim. What do I do?