[iamelgringo]

Giving a person 50% of a company that's vested over 4-5 years is much different than giving a person 50% of a company the day the come in.

If you come up with a decent vesting schedule, I think that you can probably avoid a lot of the pitfalls that you've described: What if he screws it up?

If he screws it up, fire him, and let him walk with the 3-5% he's earned in the 3-6 months that he's been working on the product.