You could contact Amex to find out if there are upgrade offers to Platinum not showing on the website. It'll never touch a true welcome bonus for new cardholders, but you do get to keep the account age. I did a product change on mine from Gold to Premier Rewards Gold. The big difference there is that PRG has a $35 higher annual fee but does come with a $100 airline credit. Downgrading to green is likely a waste of $95.
You mentioned he is upgrading to Delta Reserve at the end of the year. Amex will sometimes offer MQM bonuses for the upgrade from Platinum SkyMiles, so it may be worth considering whether that would help more with Medallion Status if it's credited in 2018 or 2019.