Don’t assume that because he’s not putting pen to paper that he’s not working on it. He may be knocking around ideas and possible constructs in his head.
I have had success by taking stabs at things (with a pen in hand) until the false starts knock something loose, but I also have found that if I allow myself some time to think things through - in the shower, on a run, on my drive to work -’ things sometimes break loose too.
Wishing you luck. (And loving @scorekeeper1 'a advice about parenting as though it’ll all work out. It shows your kid that you trust them as well as what optimism looks like. Nice gift from a parent!)