If OpenAI, Tesla and Google cannot retain him, then probably nobody can. Probably he'll be doing YouTube videos all day long.
To attract someone at Karpathy's level you would need a project that is both wildly challenging (and yet not the typical startup "challenging" because it's a poorly thought out idea) and requires the kind of resources (compute, data, human brains in vats, etc) that would make your place look far more interesting than OpenAI.
But, hardest of all, you would need startup founders that could tame their egos enough to let someone like Karpathy shine. I haven't talked to a Bay area startup founder in a while who wouldn't completely squander that kind of talent by attempting to force their own half-baked ideas on him, and then try to force him out months later when he couldn't ship those poorly thought out products citing lack of "leadership".