Your boss however good his intentions is bound to be thinking also about his organisation without you so you have to factor that in - why not ask him for a year long sabbatical and also work out a way that you can possibly still be there to answer questions if they get stuck in your absence. If the company that you are with are this worried about you going then it is probably time they faced up to not building a business around one person and also you can help them transition to to time without you there