I say make a mod for a popular and easily moddable game like minecraft or roblox. You will have a 3d engine and content for free - this is incredibly important.

I say make a mod for a popular and easily moddable game like minecraft or roblox. You will have a 3d engine and content for free - this is incredibly important. You need a quick MVP or he will lose interest. Then you can try to gradually add code and assets to make it closer to his vision.