Certainly sounds familiar, my tip to him is to try and write recipes down and get in the habit of mise en place-ing (prep chopping / pre-measuring) when you know what you're gonna cook. Once the food hits the hot pan, any semblance of a plan goes out the window

(but also know "sticking to habits" is hard for us with ADHD, as it frequently goes against our nature, so don't be shocked if he struggles there)

I tend to be the one to cook the "whatever's leftover in the fridge" dish, which is a guaranteed source of a little chaos. In those instances it's always helpful to have my wife around to pass ingredients or do some prep tasks on the side so I don't lose focus and burn the onions.

Also, if you don't already have recipes written down, having someone help build out a recipe book as you go can help smooth out future cooks.

Shout out to Recipe Keeper - after a first cook, usually from a website or book, we put everything we like in there for future reference.