If this person is so critical, why would you not bring them into the fold and make them an employee or co-founder? Because you really are setting yourself up for disaster when they can just walk at anytime.
I've thought about that, but he's an employee at an agency in Russia. I doubt he can afford to work for sweat equity, if I happen to run out of money, and it also might be considered a bit unethical to poach a developer from an agency.