According to Osho, the kindest way is either don’t go now—or, if you go, go openly as a sannyasin. Avoid hypocrisy; meet your father’s anger with love, humor, and patience, let him vent, and keep the atmosphere light. Sit with him, reassure him you’re happier and freer, and teach him simple meditation (vipassana). Your authenticity and joy—not argument—will make him feel good.
Be honest about being a sannyasin, stay loving and playful if he’s upset, and show your happiness—maybe teach him to watch his breath.