Well, you both learned something here. Whether this is swing or polyamory...
Apologize for the ding. Ask for his forgiveness since it was not intentional, and ask if you can make amends. (To self, listen to gut to slow things down more next time. Your gut was trying to tell you something important.)
Let him know you are sorry he hurts, and you feel bad for that. But since there was no map to follow...well... there was a ding. It def was not intentional, you are a newbie too. Let's move it forward and discuss then so next time it be less of a ding to him. How would he like the "reconnect and digest" time to be like? What does he need in the recconect and digest that you can provide? How about you? What do you need from him? Where is the happy medium? Negotiate and make a new plan to take this into account.
1) Some "reconnect and digest" time is still going to be needed with established BF in between your dates with the new partner.
2) Some clarification if this new partner is going to be a sex playmate, a friend, a romantic partner, or some mix and match thing as it unfolds. (Sort out your model and vocab so you are on the same page. )
I am not clear if this is about casual sex on the side. If so, that seems more like swinging. In this article on open relationship models
-- that's a 1a perhaps? Is that what this is and what you are after? Are you and BF clear on what your open model is after? Make sure you both are envisioning the same thing.
3) How you and BF plan to digest #2 if NOT envisioning the same thing. May take a dating time out to get sorted first.
4) How new man will digest all this and sharing your time/attention with BF. (You are now the shared sweetie, so you deal with that with him separate from BF once you are sorted? What are the new guy's wants, needs, and limits here? What does he think you have with him?)
5) What sort of rship does BF want to have with his new metamour person if any? Just polite distant people -- if other guy calls and BF answers he is just basic polite and passes the phone to you to deal with? Does he want to be friends? What does the other guy want in this department?