There is no universal federal registry, as treatment of sex offenders vary from state to state - hell, it can even vary from county to county in terms of what information is displayed or shown depending on what level they're assigned to, which ALSO varies from state to state and county to county.

You can find databases online where you can search for them, but this is because they're compiling multiple different sources together. And that isn't taking into account the ones that actively scrape others for data, which can be an absolute pain in the ass for various reasons.

If he's so bent out of shape the way Nebraska treats sex offenders, the stupid fuck can go move to California. They love sex offenders there.