Gender Health Query - a website by LGBT people who dispute transgenderism and gender ideology and want to promote safer more humane, and more effective ways of treating people with gender dysphoria and identity issues - is a treasure trove of information.

https://www.genderhq.org/trans-youth-controversial-schools-lgbt-science-dysphoria

The websites of two groups set up by concerned parents are also treasure troves of solid, well-researched information:

https://www.transgendertrend.com/

https://4thwavenow.com/

More excellent resources here pertaining to young people of both sexes here:

https://inspiredteentherapy.com/wp-content/uploads/2018/11/11.19-Gender-Critical-Resources.pdf

And here: https://gendercriticalresources.com/Wiki/doku.php?id=resources:for_parents

And here: https://lilymaynard.com/

And here: https://www.peaktrans.org/children/

There are lots of well-researched, well-argued works out there that debunk many of the specific claims that TRAs make, such as the fake stats they cite about suicide and murder rates, about male & female brains being noticeably different and trans people supposedly having brains of the other sex, and the lie the that if a young person doesn't immediately transition, they will likely kill themselves ("better to have a live daughter than a dead son") and so on. Here are just a few:

https://www.womenarehuman.com/debunking-the-myth-of-gendered-brains/

https://fairplayforwomen.com/suicide/

https://quillette.com/2019/12/07/are-we-in-the-midst-of-a-transgender-murder-epidemic/

https://sue-donym.medium.com/the-transgender-movement-and-bad-stats-a-debunking-compilation-31760947b382

If your cousin is a male who has "come out" as trans at 14, which is long past puberty will have started, and he is attracted to females sexually, then he has a distinctly male paraphilia (new word for what used to be called a perversion) called autogynephilia or AGP.

AGP is "the propensity of a male to become sexually aroused by the thought of himself as female." AGP always starts when adolescent boys begin masturbating while fantasizing that they are female, often whilst wearing, rubbing themselves up against and/or cumming into articles of female clothing or related to female hygiene such as menstrual pads.

Males who develop "gender dysphoria" and start claiming they are trans after puberty has begun do so because they have augtogynephilia. Even males who are primarily sexually attracted to other males can have AGP too. Sexologists Ray Blanchard and J Michael Bailey explain male "gender dysphoria" that comes from AGP here:

https://4thwavenow.com/2017/12/07/gender-dysphoria-is-not-one-thing/

AGP is especially common amongst adolescent and full-grown males today due to the predominance and easy availability of genres of internet porn and known as "sissy," "bimbo," "forced feminization" and "hypno sissy." More on that here:

https://grahamlinehan.substack.com/p/sissy-porn-the-gender-movements-dirty

More on autogynephilia here:

https://quillette.com/2019/11/06/what-is-autogynephilia-an-interview-with-dr-ray-blanchard/

https://sue-donym.medium.com/the-elephant-in-the-room-dc822144a81b

BTW, if there are any females living in your young cousin's household, or he visits other households where females reside, all those females need to start locking their bedrooms and other places where they keep their underwear, nightgowns, bathing suits and other "female coded" clothing, as well as other female-associated items like menstrual products, makeup and hair grooming tools and supplies so that your male cousin can't get access to them. Coz guys with AGP really get super aroused by violating the boundaries of girls and women they know, and to a boy and man they all start out by stealing or borrowing the clothing and other personal items of females in their lives to give themselves "gender euphoria" (code for massive boners) and to use as masturbation aides.

The good news is that autogynephilia does not have to overtake your cousin's life the way it so often does. Some men with AGP decide not to indulge it. Here is a lovely young man explaining his AGP:

https://youtu.be/hG9jBXGPnxE

A sexologist interviews a young man who took cross-sex hormones and T-blockers and decided he was trans after developing what he says was "rapid onset gender dysphoria" and autogynephilia that he says he's now outgrown:

https://youtu.be/NHLPk1JrPzQ

Another young(ish) man who "transitioned" for a while due to autogynephilia, which he now has come to believe was a symptom of his autism:

https://youtu.be/z6LI_l-hmQs

Another young man with AGP explaining his paraphilia in an interview with a TIF:

https://youtu.be/dz2MFYvD3rU

The bad news is that males' AGP is very deleterious and often profoundly damaging to the women in these males' lives, be they the fellas' mothers, sisters and other female relatives, and of course their GFs, partners, wives and daughters. And letting AGPs do as they want is very bad for society over all, coz these men will ride roughshod over girls and women's boundaries, they want to invade and dominate female-only spaces and sports, and they are hell-bent on rolling back women's rights to speak out and oppose them in any way:

https://youtu.be/i5k2osnQ_tc