He has witnessed the fact of him personally seen and verifying the evidence of a round earth but can't provide it.
Actually, it's more like I no longer consider it worth my time to present it to you guys; I've read enough signs that say "Don't Feed the Trolls" to understand that one shouldn't feed the trolls. Like I said, I usually just come here to be amused by the antics. And observing the circle-jerk of you guys claiming I simply don't HAVE any proof is very amusing. 
So you don't have any evidence to support your claims?
Didn't think so. Typical noob.
More like I already know, having debated with some of you before, the pointlessness of presenting it. If I say that I've been in space, then I'm told that I'm part of the conspiracy. If I present flight plans, geometric facts, physical studies, geographically tracked items, pictures, etc., then the trolls here call them fake, tell me my trajectories must have been off, or claim I am, again, part of the conspiracy. If I remain silent and chuckle, them some self-righteous troll takes my silence or refusal as inherent proof of a lack of proof.
Figure in all of that, and then remember that my main reason to come here is to watch obvious internet trolls pester each other, and why would I even waste my time feeding the trolls? When I already know that NO form of proof will be accepted, why would I offer any? That's not even why I'm here. I'm not here to debate the truth of a round Earth, nor point out why the Earth is easily proven to be not flat. Most of you guys already know this, the few that don't will regard it as conspiracy, and the rest of you will simply use it for trolling-fun.
Doesn't sound like much fun for me. I'd rather watch and chuckle.