The real question is “what is the property worth to your neighbor?” He’s dead in the water without your end. If this project develops the way you foresee, your side is going to be way more valuable. If he cuts you off, you can just extend the runway in the other direction.
If I were you, I’d be leasing your neighbor’s end of the runway instead. Or better yet, buying it. If there is a falling out (and there will be) it’s going to be a mess if all this isn’t formalized.
I’d be forming a legal partnership vested in a development company. Apraise and then sell or cede the entire runway & areas to be developed to the partnership. More equity could be developed by your partner, since his part is smaller, by developing an FBO & fuel depot on his end, for instance.