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So honestly, almost any proper desk chair is going to be more comfortable for long sitting sessions than a rigid tall dining chair. Like quite literally, he'll be better off in a $40 office chair from Staples compared to what he is in right now.
Now, in terms of "THE BEST", Herman Miller makes phenomenally ergonomic chairs that are quite frankly worth the ~$1600+. But obviously, that is outside a lot of people's budget.
SecretLab makes "GAMER CHAIRS", but they are also phenomenally ergonomic and comfortable chairs. If your dad is a larger man, the Titan (actually apparently on sale right now for $369) is truly excellent. it is what I have, I probably spent 10+ hours a day sitting in that chair the last year and my back feels great.
Otherwise, other users have made great recommendations re. office furniture liquidators. I mean seriously, basically any office chair of any sort is better than what he has now. The more "fitment" options on the chair (recline, back support, etc) generally the better, cuz he can just fiddle with it until its comfortable.
The cheapest option I can personally recommend is an Amazon office chair, link: https://www.amazon.com/Office-Chair-Adjustable-Ergonomic-Executive/dp/B08JGGJZY2/ref=sr_1_35?dchild=1&keywords=office+chair&qid=1624307652&sr=8-35
This used to be an Amazon Basics chair but it looks like they've rebranded it. Its like $110 usually. I had one at my home office for years and loved it and got one for my office-office, but when I was sitting in that same chair for 8+ hours at work and another 4+ hours at home every day it started absolutely killing my back. But, it was solid as long as I kept my sitting under like 6-7 hours in it.
I found my office/gaming chair on Amazon, I allowed myself to scroll a few pages into the suggestions and found the one I liked. here