considering how young he was when he died and all the research you did, it almost certainly wasn't anything you could've prevented so try really hard not to beat yourself up wondering what you could have done differently! you shouldn't have had him for more than a month since they shouldn't be re-homed until 8 weeks, so it was probably a pre-existing condition... and if you got him younger than that, shame on the breeder. (heck, it's illegal to sell them younger than 8 weeks in many states)
you should definitely get yourself another bunny (or better yet, two!)... and if I were you, I'd get them from someone different this time.