Can someone briefly explain if there is old lore around him crashing and going on to do the rest of his work?
This is actually a tricky question. There is almost no old Lore about
John Jameson. We know that JJ's son (the player from FE2) was
also named John Jameson. Source: The FE2 manual, the will of Peter Jameson*:
https://media.discordapp.net/attach...-11-28_at_7.51.17_PM.png?width=307&height=464) You can see there the will is addressed to J. Jameson Jr.
So, I'm not aware of "john" being mentioned anywhere specifically in old Lore - the first name of the Commander was left open iirc. But putting together the pieces we can see that we have Peter Jameson (unknown, died in 3199 (FE2 manual)*), father of John Jameson (died in 3150 (ED crash site)), grandfather of John Jameson Jr, (pilot in 3200-? (FE2 game)) --- unknown.
I haven't been able to track down a scanned version of the FFE manual to check to see if there's a "will" in that, and what the names say. In the online version (the one that's bundled with modern ports of FFE) it's actually a player-made manual that seems to use the exact same "will" from the FE2 manual - I'd be really interested to see a scanned FFE manual to see if the will is different (or exists at all).
It's definitely worth noting that it's entirely possible that "John Jameson" is Peter Jamesons, brother, uncle, second cousin, clone, Life Model Decoy... - we don't have any real reference other than the "will" in FE2, so take with a massive pinch of salt until it's confirmed one way or the other in ED
Was he the guy who delivered the antivirus or just the guy in the first game?
in FFE the "antivirus" was delivered (and not) around 3251 by the player (so 100 years after the JJ Cobra crash site, if we've got our timeline speculations correct) - However this is FFE Lore, we have zero ED info about this event (if it happened). So far, all indications are that John Jameson in ED (the guy in the crash) is supposed to be the "guy from the first game", so the crash would have taken place in 3150, or 3151, which we know was the time when the Thargoids "vanished" according to the Sarasvati logs.
BUT! Most of this is supposition based on piecing together one word or one line or text mentioned in various places, so best to check all sources yourself and see if you can develop a better idea or verify what's there.
*Peter Jameson is also part of ED Lore: "The most coveted was the 'Elite' badge, and the first of these was awarded to Commander Peter Jameson, who wore it on the rear of his Cobra MkIII for the rest of his life." from Tourist Beacon 0245 - Shinrarta Dezhra.