You have thrown up a couple of interesting issues in your post and I would like to offer some reassurance.
The passage of time is rarely kind to us, at least as far as our appearance is concerned. Apart from the general ravages of age, we have to deal with the effects of testosterone conspiring against our inner woman. But it's not a lost cause - both you and Marion talk about the benefits of well applied makeup - what about arranging for him to have a makeover with someone who specialises in MTF? The results can be spectacular but check their websites first as there are a lot who don't understand that it takes rather more than foundation, eyeliner and lipstick to change a male face to a convincing female one.
Secondly, it will be very beneficial for him to feel part of a community, even if he doesn't socialise as a female. Susan's is a great resource for the transgender community but of more limited value to crossdressers. These days, I spend much more time on a photo sharing website where there is a vibrant crossdressing community and where I have formed some fantastically supportive online friendships. I'll never be a contender for Miss Universe but there's nothing like a few compliments on a photo to make a girl (if someone of nearly 60 can be called that!) feel good!
Finally, let's talk about 'passing'. For the reasons I gave above, it is very difficult for a male to look like a cis-female. Of course, photographically there are all sorts of tricks we can use - weeding out the 90% of photos that make us look hideous, decent lighting and digital enhancements such as Photoshop and FaceApp - but living in the three dimensional world is a different proposition completely. I can think of YouTubers who look fabulous but give the game away as soon as they walk or because their clothes do not match the surroundings and I can think of others who, facially, are not in the top flight but would blend in anywhere because they understand everything about appearing female and not just the facial aspects.
We can all remember times when we've seen someone who is obviously trans but I often wonder how many trans people we walk past without noticing because they've understood how to blend in. With your help and support, I am sure that your brother can be one of those people and, as long as both of you start to think about how he can blend in rather than giving up because of a perception that he no longer passes, I'm sure he's going to be absolutely fine.