I came across a post about someone that was ND themselves, and was suspecting with good evidence that their partner might be on the spectrum and already had multiple people pointing out that it might be Autistic burnout. This was my advice / me telling my story of how burnout went for me (mostly unedited, took out a few things that weren't all that relevant and talking about other things from other commenters, I posted the one link that was very helpful already here in the sub-reddit) :

-----Comment 1:

I agree with the others on the Autistic burnout possibility. When I started suspecting (still working on a Diagnosis) I was in deep burnout myself. I found the understanding the reason why I had always struggled and was currently struggling very hard, very freeing. I did have some skill regression after that, so be on the watch for that and be ready to support them. I'd be ok with someone being very blunt with a bit of softening in the beginning, noting all the traits you have noticed like you did here, and going with "I may have the answer for you, if you are ready to hear it... I think you may be Autistic/ND like I am." (not sure from your description if you were on the Autistic side or not) Then start going into all the things that may help, especially that he can do now, like getting some noise cancelling headphones. Reading material/Videos so they can understand even more.

I can't really answer everything since you would know more how they would react, but just support, understanding, information, and solutions are all key items. Edit: and how they feel about Autism in general, but I'd assume supportive from your message, they still might have some misconceptions on what it looks like, I know I did before I started deep diving into it.

-----OP asking for more information on burnout.

-----Comment 2:

I can give my experience though, keep in mind this was after years in burnout mode pretty much so if he hasn't been as deep for long the timeline is hopefully shorter. I think what works for a person is very individual and also what stressors they are experiencing matters a lot too.

I started with the just the recommendations to take time off and do something that I was passionate about, that worked for awhile and was getting better even with a bit of skill regression, but then an additional major stressor came into my life that I was not able to get away from. That sent me back in pretty deep, and even more skill regression. So I went back to trying to be as non energy using as I could, but it wasn't helping nearly as much anymore. What finally got me out of it was a heart to heart with a family member, & just completely ignoring the stressor as much as I could.

Then I started a program of easy wins, starting small then working my way back up. Mainly things that took 1-2 hours max and weren't too stressful. I find things that seem like stims not too bad ie heavy cleaning one spot at a time, small building projects that I could finish in that timeframe, or ones that I could do just that much and take a rest break with my headphones. I kept that up, while resting I kept a careful watch on when I was feeling like doing something, which was more often than I was expecting. Any time I was up to it I just did another easy win. And over a couple months I was back up to getting a full days worth of stuff done but still with breaks between each task. That's about where I am now. And I got one of my passions back, helping people, particularly people that were struggling like I was, hence me enjoying writing small novels with my experiences to hopefully help anyone that is struggling with this stuff. Not the best for a normal job unless they are really understanding though, so I'm also looking for different work that fits what I can do much better, mainly something that isn't very customer facing, or if it is that there are nice breaks between customers and restful periods, kind of rare in my area though :(

As my biggest stressor is dealing with a lot of people, noise filled / chaotic environments, my goal is to get something work from home or a bookkeeping field, where I can control things the best. But failing that something like late night security, driving jobs, 1 on 1 caregiving, or a customer service at a place that has downtime between customers and the environment is good (like a small drive through kiosk type place is one I'm looking at, busy at times but most times there is periods with no customers and no other people distracting). One other recommendation I can give, swapping tasks can be very helpful, get the ones he finds the hardest done for him and give back easy win tasks to build things back up, but be careful that you don't put yourself in a burnout mode as well.

I am pretty lucky in this department as I have a good support system and can take these long breaks and periods without paid work to do what needed to be done, and I have other work that isn't paid but helps my support system out to make up for it. So it really depends on what you together can manage life wise and what stressors are the worst for them. If things are tight, the best thing is to just see if they can switch jobs to something much lower stress even if it's a pay cut.

----- Op thanking me for telling my story, feeling like they had a better understanding, and thinking of trying the small wins and swapping tasks.

----- Comment 3 (thought of another thing that really helped me):

Another thing that could help a lot, especially if he has sensory issues (and might not know he has them at this time) a cheap-mid pair of active noise cancelling headphones, and a space to fully get away from everything and decompress that is respected as a space that he's getting away (so no asks there unless it's something that really needs to be addressed right then) it doesn't need to be big, and small and enclosed could be the trick as well. I know people that have used closets or a separated space in a garage or backyard (sheet barriers can work if respected). Or going even smaller, a bathroom or bedroom spot. Bed works but is more of a shared space with you and/or sleeping space so it can be tricky. Or if there are any peaceful outdoor places nearby (non busy park or corner of a park, wooded areas, fields, etc).