Laws vary by state but I don't see this as any different than if he knew exactly where that buck was when it died. If it crossed a property line he didn't have permission to go onto, he's shït out of luck. I've heard some states (I have no idea about Iowa) will fine a landowner for not allowing someone to cross their land to recover a dead critter: if they know it is there....but that doesn't sound like the case here anyhow.
Based on your description, let alone the 5 year timeframe involved I see absolutely zero legal standing for your neighbor. Even less because it is "believed" to be the same buck.
