There's this idea that short men need outsiders (like tall men) to speak against heightism, because any talk from short men is taken as bitter whining. Aside from the shaky odds of this happening, I disagree with the notion.
Now, that's not to say outside support isn't needed, it's just needed for a different reason.
Take a look at the forms of discrimination that are taken seriously (at least by enough people): racism, sexism, homophobia, ableism, fat-shaming, slut-shaming, etc...
Notice how they all affect women? There's no way fat people are a logical priority over short men, since obesity isn't a genetic trait like height, but people care more because fat women are affected. Yes, heightism affects women too, but let's be frank here: it's mostly short men who are targeted, and it's mostly short men who oppose it.
See, this tall guy here was taken seriously simply because people take tall men more seriously, not because he was an outside force. One could claim that it makes no difference so long as people listen, but the very image of a tall man swooping in to save people is a heightist's wet dream.
I'm not saying support from taller men is unwelcome (it'd be appreciated), I'm saying two other things: firstly, tall guys can help but they must not lead because it's a counter-productive bad look, and two, they're far from a necessity because it's not the outside support itself that matters, it's the fact that society only cares when women are involved. Take the phrase "women and children" for example. Women always take precedent. I believe feminists like to call this "benevolent sexism." I call it "lucky for you." Here's an example of people being offended when short women are said to be like little girls.
Speaking of feminists, I have never seen anyone on the fence about feminism who changed their mind the moment a man championed the cause. If anything, people take male feminists even less seriously, despite them being outsiders who have nothing to directly gain. People either think male feminists are in it to get laid, or they have low testosterone or whatever.
In summary, the important thing is getting the support of those who have societal influence. I've seen others saying things like, "Marginalized groups have always needed outside help, because otherwise they're seen as whining, like how black people needed JFK." This is wrong. They didn't need JFK simply because he wasn't black, they needed him because he was a president, and white people had more social influence. Tall people and women have more social power than short men. Now the question of getting them to care about heightism is a whole different matter. My stumpy ass is stumped there.
Update: I came across a quick example of an average height guy who defends short men. He points out how people are angered by him anyway. Since short people usually get "you're just mad because you're short," it'd be logical to assume that an average height guy could change people's minds. The problem is that while an average height person doesn't have the baggage of a short one, they don't have the influence of a tall one either. When people hear "tall," it conveys something more powerful to them. When they hear a guy is average height, they of course picture an average guy lecturing them. That's what matters.