Honestly @Captain24 I think you nailed it!
@Shaz51 I think for myself, when I'm so utterly exhausted from everything, I can get a lot of decision-fatigue. Like if I know I gotta do the dishes and the vacuuming and my emails and and and.... then I get stuck cos I dunno which to do first, can't discern in my brain what is most important or most impactful, heck I can't even decide if I want toast or a crumpet for dinner!
What helps me a lot is having someone 'take charge' - I don't want to be asked what I want, cos I literally can't pull anything certain out of the din in my head. Having someone say 'here, I know you're exhausted so I've put your dishes away for you since they were taking up space' or 'I reckon you're probably hungry, I'll get us some dinner - pizza ok?' or like even reminding me of stuff cos I get so 'locked on' to a task I forget that I may not need to do it right now - like 'hey I see you're getting distressed about writing that email. Do you think it can wait till tomorrow? I'll read over it for you then' - omg that is always amazing.
And when I say 'take charge' I don't mean 'do everything for him' cos a) he still needs to feel a sense of agency and b) that is a shortcut to you ending up burnt out your own self! But just like @Captain24 identified, getting asked a lot of questions or being given too many options can itself contribute to the overwhelm.
Remind him that rest is not earned, it is as vital to life as the air we breathe. Resting for us hyperactive types can be really hard cos we can't typically do nothing, but it can be more like 'mental rest' by engaging in activities that feel automatic (i.e. where we might get into 'flow-state'), reducing the amount of day-to-day decisions that need to be made, and setting aside time to just...kinda be a mess. Having a big cry can do wonders!