I am an American citizen. My co-founder is German. What do the steps look like to get him eligible to work out of the US (say, if we're accepted to YC) and how

I am an American citizen. My co-founder is German. What do the steps look like to get him eligible to work out of the US (say, if we're accepted to YC) and how does the company need to be structured to make it as easy as possible? Thanks in advance!