Freedom of movement is still in effect until the end of October, as for after that time the visa/residency question is up in the air but there's an extremely good chance residency of some form will be offered to British citizens who are already established in Germany.

As for the company being incorporated in what will become a third country, from my research I can't see any reason why that will fall afoul of any regulations (as long as everything is properly reported to the authorities).

I think the "only" real change is that he will need a work visa, the other aspects should remain the same. A EU citizen can freely choose his workplace in the EU (which is a reason why the British voted exit, they specifically didn't want that).

Thankfully freedom of movement is still in effect until the end of October. So no visa's required just yet. As for residency after the brexit date that's completely in the air but I would rather be already inside germany than outside.