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Meta now has Teen Accounts for those aged 13-17 that are set to protective Teen Account settings which by default restrict friend requests.
Settings can be changed but parent and teenager enter into an agreement over what these are, what the teen can and can't do including who they are allowed to befriend and communicate with.
Parents will receive a push notification if their teen changes any of these settings.
In other words, don't try to do this without the mother's consent or you could make a bad situation worse.
I disagree. Unless the mother has explicitly told the OP and her son not to contact them, I don’t see how attempting to reach out can be worse than no contact at all. Worst case scenario OP is told by the mother not to do it again. Best case is that she allows some contact.
This is all provided OP’s son wasn’t an abuser who was enabled or protected by his mum. This is the only scenario I’d advise against contact, for the sake of the children. They would be better off with no contact in such a case. If no abuse took place, shoot your shot!