Apologies for being a little late to this party but I would like to give a slightly differing perspective to some of the other respondents.
Firstly, let's state the obvious. There's probably not a day that goes by without your brother wishing, with every fibre in his body, that he had been born female. He'll look at women in the street with envy. He'll walk past a cute outfit in the window of a ladies' clothes shop and wish he could go in and try it on. He'll fight with the urges to set the 'inner woman' free. And when he does get the opportunity to dress, he will feel sensations like no other; his worries will evaporate and everything will feel right. And I know this because I've just described myself!
But there's an important point here. Wishing one had been born female is very different to wanting to transition to become one. I would be lying if I said I have never considered transition (I have gone on record on this forum saying this) but I am realistic enough to realise that transition is not for me.
But it's not easy - you make the point that your brother's feelings seem more intense now and I think that practically everyone who posts here would concur. These are urges that don't go away and intensify as you get older. For some, transition is the answer but, for others, it most definitely isn't.
And I think your brother's assertion that he would rather be single than have another relationship where he could not dress again is one that I can definitely identify with. I am married but rammed well and truly into the closet by a disapproving wife. The guilt about what I do is all consuming and I often dream about being single so that my female clothes could be hanging in the bedroom wardrobe and I could change whenever the fancy took me. But I think that throwing away my marriage and everything that goes with it is too high a price to pay. Your brother has the chance to live that life and there's nothing wrong with that. Who knows, though - maybe he'll meet a lovely lady who can support and encourage this side of his life?
So I wouldn't worry too much about this. Your brother has the chance to live his life as he wants to. Maybe he'll be content just to dress at home, maybe he'll want to spread his wings and socialise as a female. Perhaps, in time, he'll decide that he wants to take this further and then, as others have suggested, he will need professional guidance.
In the end though, you obviously love and support your brother and that's what he needs more than anything else.