Since there are so many less than optimistic answers here, I'm going to suggest something different.
With your brother doing his hobbies, if he gets really good at them, like wood working, model building, sculpture, whatever, he might be able to turn that into a decent small time business.
Personally, I'm trying to transition away from a day job to the business I run at night, which is basically using my hobby skills to make and sell things on Etsy. In fact, I'm currently working on buying a larger small business in a similar industry so I can merge with it and do it as my only job. Also, I'm trying to do it on Much less than $1m.
It'll be hard work for me, and your brother if he goes this route, but it's possible. There's a very real and huge difference between work you love and work you do just for a paycheck.
I have a feeling that your brother really hates his job, even if he doesn't explicitly say it, and/or may feel as if that line of work is a dead end.
My dad doesn't understand how I can give up a job that pays as well as it does, and I find it hard to believe I'm doing it, but I've set realistic goals for me to hit before making this plunge and I'm going for it. It sounds as if your brother has made goals and is also going for it, too. Go Him! I wish I was able to hit goals like his.
There's another possibility, one where your brother gets bored after 1-2 years of retirement and goes back to work. It happens.
Really, the worst thing you can do is to fight him on this. It's his life, even if there's a fiance involved, and you need to accept his decisions. He is an adult and has been making decisions for himself for quite a few years as an adult. You just need to accept this as another of his decisions. Fighting him will only make him want to retire more and stay retired, even when he knows it's the wrong decision. He'll want to "prove that he can do it", even in 10 years if his marriage is failing, his car and house are being repossessed, and is on the verge of bankruptcy.
There's also the possibility that he could retire now, sell those books of his, which 1 might become a best seller, and not have to worry about money ever again.
It's way too early to say he will fail or succeed, so just roll with it.
I would, however, avoid loaning him any money. He made his decision and needs to live with it. Of course, if it's life and death, give him some help, but never rub it in his face. Offer suggestions, but also never make ultimatums or try to tell him what he has to do. Those just make a bad situation much worse.
I've lived off less than $40k a year (before taxes) most of my life, and I'm assuming your brother doesn't have the student loans or the credit card debt I had.
My best friend takes home about $40k a year and is a singe dad with 3 kids.
It may not be comfortable, but it's doable.
Something I haven't read in other answers is any calculations on how much the OP's brother is currently saving.
I did some quick calculations, and even saving $50k a year, it would take 19 years to save $950k, so this brother is saving much more than that, given the likelihood that he hasn't been saving an equal amount each of those potential 19 years. It would take almost 10 years to save the stated amount by saving $100k a year.
Since the OP stated that the brother expects to retire next year, that's as little as 6 months away as well as $50k away from the goal, so this brother is definitely sacking away the money, instead of spending it.
Of course, this doesn't account for earnings on stocks/bonds, real estate price increases, and probably a host of other possibilities, but this brother seems like he's already a frugal person.
I would hazard to guess that he is already living on $40k to $50k a year. Removing the need to pay for constant transportation to/from work, work clothes, work lunches, and all the other things that cost money and are work related (morning coffee), he probably can get along pretty decently continuing to live at $40k a year.
I remember at least one job I had where I thought I couldn't afford to continue working for that company, since I wasn't getting paid much more than I was spending to remain working at that job, including all the other bills I was paying.
At a position where he's making $150k a year, he might be expected to wear expensive suits, ties, shoes, drive an expensive car, eat at expensive restaurants, etc. Those costs can really add up to massive savings when they are gone.
As an added tip for the brother, even if he's already frugal, is to get a copy of "America's Cheapest Family". I'm in no way connected to the book or getting paid for it's advertising, I'm simply another satisfied customer. I've used the ideas in that book to help me remove debt, stay out of debt, and remain sane.
The family who wrote the book has 5 kids, bought several houses & cars, sent kids to college, taken family trips, moved across the country, and so many more things on not much more than $40k a year income.
I still say early retirement is doable, it just takes self control and planning to be able to make it happen. Unfortunately, those are two things most people don't have/do/want/can be relied on for, so we just assume that no-one has it.