I bought this one when I was 462 lbs. Four years on and it's still my favorite chair and it's held up like a champ, it's my main office chair. It was a little spendy but totally worth every penny. I would easily have spent that much and more on cheap chairs in the four years that I've had this one. Bonus is it has an adjustable vibrating back cushion that still works after all this time that I have really enjoyed.
I bought this chair when I was at my biggest. Spendy but I've had it around 4 years now. Well built and comfortable and has a little adjustable back massaging pillow.
Also, I agree with others, get up and move around every hour, your circulatory system needs that.
Furniture companies call it "chair and a half".
But if you want more of a computer chair, then look for ones like this perhaps: https://www.amazon.com/dp/B07Q48H53D/
Obviously it depends on your size. You could be a 350 lb person, you could be 85lbs.
I wouldn't suggest a beanbag chair - they're hard to get out of, and if he pops it the pellets will get everywhere.
You need to tell your husband that he needs to have a frank discussion with his cousin, tell him that he needs to buy a chair that will work for his weight and bring it to your house. I found this one on amazon.
He also really should repay you for all the furniture he's broken - it's got to be quite a bit by now.
I've had this for almost two years and it's still in great condition, especially considering I never just sit down, I collapse because of my knees. I combine it with a purple cushion and I'm a happy booty remote worker.