Sorry to hear your DH might be heading into burnout, I'll share my experience of what I've experienced in the past and also how it presented in DD and how we helped with recovery. Hopefully he can avoid full burnout if he makes some changes now.
DD basically just kept going and going (and masking) before hitting burnout, almost like she was stuck on full steam and unable to relax/stop. She was very pale looking, frequently ill and was sleeping badly, her nail and skin picking was in overdrive, she zoned out a lot and often looked blank or not quite with it for moments at a time. Very rigid thinking around routine and determined to keep doing stuff even when she was probably too tired and needed a break. Then she just stopped one day and wouldn't leave the house, all she could do was watch TV and play games. Stopped drawing (something she loved doing), seeing people outside the house, no going out, no school and no to all playdates. Lost all interest in Lego or toys. Very low energy and almost apathetic in a way.
We dropped all demands, she was pretty much allowed to just chill on sofa with soft toys and familiar TV shows. We focused on primary needs like food, fluids and safety. I had to help with washing, toileting, dressing and baths whereas before she was able to shower herself and dress etc. It was quite the regression.
For me, when I've been clise to or in burnout, it's like all my senses go into overdrive and I hear everything louder, foods can become 'wrong' in texture and taste, lights are distracting and everything becomes too much. At the same time it was like I was moving in slow motion whilst everyone else was in ultra rapid and I also couldn't process what people were saying. I couldn't speak sometimes and would struggle to respond to questions.
What helped was basically just shutting down and becoming quite reclusive. Couldn't cope with socialising or going to busy places. Walking helped, particularly in the forest or in rural locations. Sitting in the garden or outside in good, particularly if practising mindfulness/awareness of surroundings. Bingeing on comfort TV shows, heavy blankets, warm baths and when we still had a dog, dog cuddles. I also have a spiky mat to lie on, scented candles to burn and lots of comfort films/books/shows/music to turn to which helps.
Spending time on a special interest is also something that I've heard is very helpful for burnout.