I hate asking this because I do want him to change but is there a chance to resteer autists once they get something deadset inside their head? I'm guessing it is, but it must be an uphill battle. You're his brother so I'm sure you and your parents have more experience getting him away from bad ideas he comes across.
I'm a sperg so I might be able to answer this.
When autistics get obsessed with something (what they call "special interests"), it's more than just a hobby. It's basically how they process the world. Everything from that point gets filtered through that obsession. It's their social life and the way they understand everything around them. Combined with autistics feeling isolated and unable to connect with society, this makes a perfect storm for trans stuff to take hold and become their special interest.
Unfortunately, there's not really a good way to snap someone out of a special interest. The only way is to replace it with something else, and even that doesn't work too well (you can have multiple at the same time, and the interest usually never completely goes away even after it's passed).
Plus, autistic people usually have black-and-white thinking, so suggesting anything other than trans shit can make them think you're Hitler.
Once autistics get obsessed with gender, that will be all they talk about forever and ever. The most well-adjusted trans autistic I know has moved on from that obsession and gone onto something else, but if the conversation turns into trans stuff, they'll immediately switch back into "gender obsession" mode.
From other desisters and detransitioners, it seems like the only way autistics can wake up is by a sudden burst of self awareness.
I think part of the problem is that autistic people are usually alexithymic (can't figure out what emotion they're feeling). Maybe helping them understand what they're feeling could help them not assume they have dysphoria.
(also for what it's worth, a ton of autistic people go through this pipeline: "I don't care about gender or what people call me" to "wait that's a sign of being nonbinary?" to "oh I'm super trans").
Sorry for the spergout (seen every autistic I know go through this).