If an employee receives another job offer and can't make up their mind, what's the best way to force them to make a decision and stick to it?

The reason I ask is, a Fortune 500 company has been courting one of my engineers for about six months. About three months ago, the engineer told me he would probably be leaving within 30 days as soon as he negotiated a contract with the Fortune 500 company. But then negotiations stalled. I've been waiting three months in limbo, not really knowing if he is staying or going, and he doesn't know either. He's been distracted and reluctant to start any long-term projects, because he doesn't want to leave us hanging. But I can't stay in limbo forever. I really want him to stay or go, so I can plan accordingly. I told him I needed an answer by the end of the month, and his answer was, "Yeah, me too." I don't think he's taking it seriously. So, I'm thinking about telling him I want a one-year employment agreement with. Either he signs it, or I'm letting him go, and he can hang in limbo by himself. Is that the best way to handle it? If so, what penalties should I use if he violates the contract? Loss of stock options? Really, I don't want him to feel like I'm pushing him into a corner. He is a good engineer, and I actually hope he stays. I just need him to make up his mind!