Grandfathering of 7 year frame re GWROBs

Some elements of this outside my familiar zone: Father (widower) wants to give away 50% of family home to adult son, who lives there too. So PET if he lives 7 years, GWROB if the son ever moves out while father is alive.

Son has own representation concerned about CGT implication. CGT implication I can see is that if son later has a different primary res, CGT (minus PRR years) due on his 50% of his gain from gift time to the future sale time.

Where I’d love some input is in father concerned about grandfathering-in of present 7 year pet/gwrob scales if father&son do it before oct budget (in case it changes). I’d assume the position is that if a GWROB was created (son moving out) and then house sold, 7 year clock starts ticking again on son’s share of the money (assuming no POAT issue generated with son’s money). But since son moving out changes it between PET and GWR, can this newly ‘created’ GWROB likely trigger new rules at that time? Or is the gift likely to always revolve around the 7 year frame?