He's not allowed but i'd hardly say it's the crime of the century if he moves back home to isolate with his old man.
Police aren't really pulling people over and there's not been any checkpoints (aside from next to well known beauty spots that people initially flocked to).
If he did get pulled by the police he could easily say that he's going home to take care of an elderly relative who's vulnerable (sorry ).
I'm not for people blatantly flouting the rules but going home to stay with your family (that's staying NOT a flying visit) isn't really that bad and is a LOT different that people who are just going out to get pissed round their mates house.