Could be Jean Luc & Beverly's son from an alternate universe. He showed up in this one when he was already a child which explains the 20 year cut-off, but a son who clearly looks older than 20.
Whatever Beverly discovered with his appearance led to her cutting all ties and running away. Perhaps there's an ongoing plot from that alternate universe to replace their counterparts in this one hence the not trusting anyone.
I hope it's not Mirror Universe Terrans although it would explain why Shaw is such an asshole and why his ship lighting is so dark. But that really has been done to death by Discovery at this point.