The honest truth is... It doesn't matter. Being a dad/father is not the same as being a genetic donor.
Your wife is not the cheater here and has no obligation to say anything. Again, because it doesn't matter.
The easy thing is to immediately tell him. The hard thing is to let the past stay in the past. If he finds out on his own means, and cares enough to roach the topic, it's a great time to have the conversation about what a father is.
So, were you all getting paid by Reddit for content and moderation? Because if Reddit wasn't paying for the service, that would make them the product too...
That saying breeds a complacent skepticism. Even if you pay for a service, that doesn't stop the provider from making you a product. Likewise, not paying may mean there's mutual benefit.