I would suggest offering him a position of power to do with what he pleases, and then do not attempt to intervene when he works. Alternatively, request a freelance contract. And again, do not intervene with his work.

[ Shittiest advise ever, BUT THE BEST WAY TO GET HIM TO WORK WITH YOU ]

I am at a loss in the same capacity, but I have spent the entirety of my existence with Mr. Stark, and it is a fair assertion that he does not respond to typical human subtleties as most do. The playing field is fairly even.