I'm a Designated Safeguarding Lead in a school.
OP, you have had some helpful advice on this thread but also a lot of advice from posters who don't fully understand the risks around online grooming and radicalisation, or it's link to real-world criminality, violence and the exploitation of children. If there is one thing we know from cases where teenagers have commited horrendous acts of violence after being radicalised by online extremists, it is that parents and carers often overlook the early warning signs. You're absolutely right to be concerned.
Your DS is particularly vulnerable due to being AuDHD. This means that he is likely to be suggestible and easily influenced, despite being academically able. He is also more likely to fixate intensely on particular topics that capture his interest. He may also miss red flags in friendships or relationships that might be apparent to others, making him more vulnerable to grooming and exploitation. There is some evidence that extremist groups are now deliberately targeting ND teens online for these reasons.
Do you regularly and thoroughly check your DS's phone and all other devices? If not, it is absolutely essential that you start doing so. Given his apparent level of interest it is likely that he is not just passively watching far-right, racist and misogynist content online but interacting with individuals or groups who espouse these views. It has been well established that far-right groups, including banned organisations who are known to commit real-world acts of terror and violence, target vulnerable children and young people on platforms such as YouTube, Reddit, Tik Tok and even seemingly 'child friendly' online games such as Roblox and Minecraft. They befriend them and groom them over a period of time and once they have established a deeper connection with the child they will invite them into private chats on another platform. This could be on the Dark Web, a gaming platform, or on encrypted messaging app like Telegram or Parler. These closed group chats become echo chambers, the content becomes more extreme and violent, and children are actively encouraged to ignore, mistrust and eventually distance themselves from their parents, teachers, and any friends who don't agree with their views. This creates a dependency on the child's new 'community' as well as leading them to become even more entrenched and extreme in their views, meaning they are potentially more susceptible to being drawn into involvement in acts of terror, violence or criminality either online or in real life.
You need to find out who your DS is communicating with, find out what sites and online forums he is visiting, what videos he had been watching, who he is following on social media and who is following him back and (this is really important!) screenshot any troubling posts or conversations. This guide is really useful in knowing what to look out for as there are a lot of symbols, code words, memes, abbreviations and seemingly innocuous combinations of numbers that are used by the far-right online
www.childexploitationeastmidlands.org.uk/uploads/signs-of-hate-%28hope-not-hate%29-erw-signs-and-symbols.pdf
When you have done this, call his school and ask to speak to the DSL. Tell them you would like them to make a Prevent referral. You can send the DSL any screenshots of concern to include in their referral. Some parents, understandably, worry about these referrals being made but please be assured they are not interested in criminalising vulnerable children. Their role at this stage is very much around protecting children like your DS, educating them and supporting them.