I'm glad to hear things are going well!

I would think that introducing him to one cat at a time would be less stressful for him. Less of the "They're ganging up on me" feeling. I would pick the most easy going of your group, the one least likely to put up a fuss, and introduce them first. That way if they hit it off, the others will see that he's not a threat to that kitty and may accept him sooner.

Good luck with the introductions - let the fun begin!