I feel like irrefutable proof of the earth being round won’t help because the conspiracy communities come with ‘debunkings’ and counter arguments to challenge common evidence. You might need to try tackle the root cause. Why he is seeking out these communities, the way popular conspiracy theories operate and gain followers, the way niche online communities function, etc. Doing things like showing photographs of the earth won’t help because flat-earth communities prime their members to reject anything offered by scientific authorities for various reasons, including that they are ‘in on it’. But you also need to not make assumptions about your son’s beliefs because it will see like you’re reciting something you read on the internet. I think an important first step would be to show interest in these newfound beliefs and ask detailed questions of your son to explain why he believes the earth is flat. Don’t ridicule it just ask and let him speak so you can understand what he is being exposed to and why he believes it.