Pick a last date with plenty of time (say 3 months). Write down your thoughts in a doc - make it detailed and touchy with reasons for leaving, a rough transitio

Pick a last date with plenty of time (say 3 months). Write down your thoughts in a doc - make it detailed and touchy with reasons for leaving, a rough transition plan, timeline. Schedule a lunch. The night before, send the doc. So that he reads the doc, gets 12-14 hours to digest and think, and then reconcile with you and work out a transition plan.