Although I have no personal experience, I can tell you what people I know have done and what is recommended by lc's here. Slow is the best way for both of you, so he has time to get used to it and you don't get too uncomfortable. If you want to keep giving him EBM during the day in bottles, you can just pump at the normal feeding times. That's what I'm hoping to do when I go back to work, until dd is a year.
But to drop daytime feeds if you're replacing them with formula, do only one feed at a time and give it at least a couple of days before dropping the next - or even a week. Every woman's breasts are different, so each respond differently to the drop of a feed. He's probably only breastfeeding twice or maybe 3 times a day other than morning and night? You can give him a bottle in place of the bf and also replace it with some cuddling or distracting if he's looking to nurse.
Your breasts will adapt to what they do regularly, be it decreased feeds or just feeds twice a day. Hope that helps and good luck!