In line with what others have said, yes, the evolution of an individual's sexuality can be amazingly varied.
I would, however, have to say he ought to be careful about how public he is about it as I have found that society in general has great difficulty accepting bisexuality, pansexuality... pomosexuality - whatever you want to call it. And the faintest whiff of homosexuality in a man may have him labelled "gay" for years, perhaps life. It's somewhat different with women as I suppose the same mainstream society that has difficulty accepting a nuanced spectrum of sexualities between "gay" and "straight" for men, finds it equally difficult to conceive of a woman who deep down wouldn't prefer to be with a man, no matter how lesbian she claims to be.
I'd like to illustrate this with my own case. The expression "gay phase" can be an unfortunate one in that it's too often used by anti-gay crusaders to "proove" that there's really no such thing as an essentially "gay man" and therefor try to undermine one of the key tenets of gay and lesbian rights ie anti-gay discrimination is on a par with discriminating against, say, left-handed people.
Anyway, I'm rambling on here. Basically when I was 19/20 years old I decided I was gay, or at least bisexual - I was, I'm afraid, very confused and told different people different things. I'd never been with a woman, not through lack of (clumsy) effort mind you. I had a number of gay flings, by which I mean none of them became a relationship. I was very open about it all to the extent of flaunting it, enjoying the notoriety; this was the late 80's, believe me, the notoriety was very real! It was also very flattering to go from feeling like a complete loser in your sex life to feeling desired and popular and so on.
Anyway, I then left the country. Had my first girlfriend, and then another and then a long-term realtionship with a woman - and so on for years. Without giving it too much thought I had, in what felt like a very natural way, discovered it was women I preferred, and I would have thought that was that....
But then, after over a decade abroad I returned home to Ireland and, completely to my surprise, found that the rumours about me had been rampant in my absence. "Did he hear about X?.. went off to Y! I heard he's married another man there!" And so on..
And some people have been positively insulting. It is one thing people do not forget, sad but true. Just one instance: I was out with my girlfriend and I bisexual guy I knew from my college days drunkenly said something like "What the f**k are you doing with a women!" There have been other examples, but mainly second-hand whisperings. It's been really annoying to be honest. You'd think people would be embarrassed to be dragging up stuff from 16 odd years ago - but juicey gossip doesn't seem to age.
Well, with that off my chest, I would simply stress again that, yes, sexuality is a complex infinitely moveable feast, but do please be careful who you tell - because it may come back at you years later when you least expect it. And if your brother does find he prefers women, a "gay" past may complicate things enormously.
I actually hope people can disagree with me here on some level though! As it is a bit depressing and I do appreciate that it is up to individuals to break the mould of prejudice...