He's not talking about a US patent, but about a problem with the US Patent Office which has a strict prohibition on patents for perpetual motion devices (and others of that kind that promise more energy out than goes in the hopper. They'd not patent an atom bomb if someone came in with an application.)
He has a quite reasonable fear that entities in the United States will simply copy his device, manufacture it and sell it without paying him royalties, and all because the US isn't going to let him patent it here.
Enrico Fermi and Leo Szilard received U.S. patent #2,708,656 for the nuclear reactor.