The fact he isn't talking about this is worrying, or perhaps he is talking to others about it, just not to you.
The best thing for him would be to find a therapist who doesn't just affirm everyone who claims to be trans, but who dives deep with the patient into their experiences, trauma, feelings, and thoughts of why they think they are trans. And to work on other mental health issues first.
Although the Gender Critical reddit is just another echo chamber, there are some good discussions/topics there that could help your son think critically about being transgender, gender ideology, what transition can and can't do, and how some part of society sees this topic.
In male to female (MtF) trans women, it's not uncommon that in the end it really just boils down to a fetish/paraphilia (autogynephilia). That doesn't mean all trans women are like this, but I've found in the MtF reddits, again yet another echo chamber, it's so obvious that it's mainly fetishist males egging others on by claiming absolute rubbish, for instance: lying to each other about how good they (would) look after hormones/surgery, lying about female sexuality claiming it's totally normal for females to get sexually aroused when putting on regular women's clothing, and that lesbians should be willing to sleep with MtFs etc. Note that both the gender critical and the 'pro-trans agenda' reddit ban people who don't fully agree to their views, so you get a very one-sided discussion either way.
If your son is exposed to both 'sides', he'll hopefully start thinking about this from all angles and come to the realisation that transition is probably not for him. He may also go on to live as a woman during his uni years, and hopefully that will help to figure out if this is something that works, or not.
Maybe you can find some good literature on gender identity and gender ideology?
I'd like to comment on Fluffythefish who stated that the younger generation is fine with all of this. I can tell you that this isn't universally true. Being trans is, although much more popular now, still quite unusual and has a detrimental effect on your social life and especially on your dating life, and also on the changes of having your own children in the future. That doesn't mean people shouldn't transition, just that they should be aware of this before doing anything irreversible. Many people still struggle to understand trans issues, and especially if you don't pass I am not sure how easy it is to make and maintain friendships. Especially dating is hard as a trans person. Most people will not include trans people in their dating pool, especially pre-op. That doesn't mean there isn't anyone interested, just that you end up going from roughly 50% of people in your age bracket being a 'possibility', to probably less than 1%. A lot also depends on personality, but lots of MtFs end up dating each other not by desire but because they weren't able to find other people. That is probably a sacrifice worth making if the sex dysphoria is daily, intense, and persistent. However, if it is instead not stemming from internal issues with sex dysphoria but other mental health issues, or a fetish, your son will probably regret making irreversible changes.
For you it might be good to do your own research/home work and learn more about the different angles to this topic. You can ask 'critical' questions in a polite as possible way to your son, hopefully getting him to think more about this. You can of course completely cut him off the moment he goes off to uni to live as a woman, but that is something your relationship with him may never recover from.
Good luck!