You tell him now before he wastes even more time of his life pretending to be a father to a kid that isn't his. You own up to also cheating around the period the 2nd kid was conceived and most likely you'll be justifiably forced to do DNA tests on all kids due to your unfaithfulness.

If you're worried about him correctly getting angry then you inform him with somebody else by your side and prepare for all possible outcomes. You can't blame him especially if he has no idea that you were cheating multiple times already