Susie, that is tough
I remember my daughter when she was 5 years old got into a rumble with a classmate because my daughter told her that santa clause is her father and the other girl did not take it well :)
My opinion is honesty, is, the, best, policy, He may get a bit mad that you didnt do it sooner, but he will get over it and will rise to the occassion and will be the best support you can ever have. However, dont tell him too much too fast, just little droplets of truth, and try to spin it in a positive light,
that is my opinion only and I stick to it :) Let me know what you decide and how it goes