You're going to have to just say it.  There is nothing you *should* do to ease him into it.  Hiding the fact that you are dating or something else like that would possibly hurt Tom even more than if you were honest, if he found out.

Tell him exactly what you said in your question.  "What you are saying is out of line and that you cannot control the decisions I make."  If he gets angry about it and won't talk to you, then that's his problem.

Speaking as someone who has once before been in Tom's place (albeit not as angry/emotional about the situation), if you were being ambiguous and didn't tell me the truth, I would have been more upset.  **I could get over the fact that you left for someone else, but not if you lied about it.**