And they could freeze all his assets.
They will freeze his assets to make it difficult to hire a lawyer
Maybe in US territory, but there are still some small independent nations that don't allow for assets to be seized from their financial institutions. Cayman Islands, Bermuda, Bahamas, Panama, British Virgin Islands, Dominica, Nevis with St. Kitts, Anguilla, Belize come to mind. All are good to great tax havens and most do NOT permit the export of personal or corporate information unless their own country has filed a civil or criminal offense against their foreign accounts.
I doubt Trump does not have his assets spread through many of these countries. BTW, forget Switzerland, they now have an agreement with the US to give names and accounts of persons or corporations the IRS deems suspicious.