You ask him questions about what he remembers in a mutually unthreatening environment (somewhere quiet and public like a park is good as social decorum keeps people calm) You ask about how it made him feel and what he thinks about it, you don't blame him for it nor try and rubbish the father despite your personal feeling as it is his father and he will just resent that and could prevent your relationship developing but you need to clearly talk about your feelings and why the situation happened and the outcome.